A civilization of land-living humanoids occupies a patch of land on a continent (patch of land size: a few US States, let's say Montanas) and, without even exploring the continent fully, develops space travel and starts to explore space. Their home planet is similar to Earth, fully habitable, with a number of continents, oceans, etc. There are other races on the planet, some of them have civilizations, but they are much more primitive. What I need is a reason, why exactly would they not explore their own homeworld, rather choosing to expand in space?
I want to ask for caution, before presenting a possible scenario: Space travel is not something a civilisation would develop in short time. This likely requires centuries for a single civilisation, if not millennia (and several cultures and civilisations), as it did on Earth. During that time, your people will likely not sit still, focusing on developing astrophysics and quantum mechanics, while they could use ships or later airplanes (which are a reasonable intermediate step between land and space travel) to reach other continents far more easily than reaching space.
However, humanity as we know has developed space travel, while vast amounts of our planet's surface are still undiscovered and a mystery namely the ones covered by oceans. This is not just the Mariana Trench, which is basically still out of reach for humans, even though we have already set foot on the moon forty-nine (49) years ago.
High pressure is much harder to overcome than low pressure and there is simply not enough to gain from investing into such technology to drive its development as much as space travel. Thus, possible scenarios would be that your alternative races are descended from organisms, such as earthly archaeobacteria and thus adapted to either high pressure living far beneath the surface of the oceans or extreme heat and lack of oxygen, living within hot springs, liquid lava or whatever you like. Investing into a technology to move through lava streams, when you have the whole of outer space, has been clear decision for our scientific community and society and could also be so for your fictive society.
They want satellites.
These folks are not much for showing up in person. They are fond of their neighbors, but as you get farther out of town stuff gets weirder. Folks smell like strange spices and look at you funny. And the outside world is full of one eyed cannibal people and giant spiders and is not a place for civilized peaceable folk. These people are homebodies.
But they very much like their shows and being in virtual touch with one another! Radio towers have their limitations and balloon towers get blown down by storms. A geostationary satellite or two would be above the weather and do very well covering their land with broadcast shows and satellite phone.
Once they get a satellite up there with camera capability it confirms their fears about the neighboring countries - chock full of monsters and maniacs. But they start to wonder - are there some places farther out that might lack scary stuff and fierce people? Maybe the moon? Maybe a satellite around the moon could check it out remotely. So it begins.
Your humanoids are the indigenous species of a planet which was colonized by aliens prior to your own industrial and technological revolutions. Imagine if alien traders had landed on Earth during the Middle Ages, prior to the European discovery of the American continents. If those traders were not governed by a Prime Directive, King Henry IV (no relation) might have found himself in possession of an FTL space ship to complement his fleet of wooden-masted sailing vessels.
Primitive doesn't mean they're not dangerous. The other races may be a match for yours, despite the massive technological advantage, because they have evolved a body which is better for such combat.
A classic example would be the Eldar vs. Orc in Warhamer 40k. The Eldar and Orc are evenly matched because the Eldar are physically frail but have great technology. There are also issues with the small number of Eldar remaining, vs the Orcs which reproduce like... well... they make rabbits look lazy.
Or your opponents simply could have studied war better than you. Consider the case of the finale of V for Vendetta, which includes one of my favorite lines:
Creedy: Bollocks. Whatchya gonna do, huh? We've swept this place. You've got nothing. Nothing but your bloody knives and your fancy karate gimmicks. We have guns.
V: No, what you have are bullets, and the hope that when your guns are empty I'm no longer be standing, because if I am you'll all be dead before you've reloaded.
There are plenty of reasons to need to respect a primitive adversary. Even the Marines and the SEALS respect the snake, and take care to ensure they don't disturb one. A cottonmouth can kill, even with all our technologies.
This question keeps me thinking. Logically your species fulfills all premises for a sucessful colonization of your planet:
- Your planet is habitable, which means cliamte and temperature won't hinder exploration
- Your planet has enough land to settle on (so not an ocean planet with a single, small island)
- The other species aren't as highly developed, so it should be easy to 'dispose of them'
- your species is in the era of space-flight, which means they should able to travel easily around the planet
- Since they try to explore space they have to have at least some kind of curiosity.
But the hardest argument against having a "not fully explored planet" is probably
- if they have space-travel-technology, they are also able to build sattelites, which would make it quite easy to explore the planet from space.
I've thought about a few reasons they might've chosen not to explore their planet
- The nations of your species are constantly at war. Even though there are only primitives, putting aside valuable resources and manpower to expand on the own planet can break the balance between the warring nations.
- Your species is pacifist, even though they'd like to expand, they just can't hurt any of the primitives, while they have no problem hurting them. So they look for an unhabitated planet somehwere else
- religious or personal believes have stopped your species from expanding on their own planet. Guess it won't be some kind of "we'll fall off the edge of the world" logic, but maybe they fear of going to hell if they die outside of the 'holy land' (although I didn't find a valuable explanation why spacetravel would be ok tho), or don't want to interfere in the eco-system.
- economic reasons. Our world is overpopulated and global warming is an immnent thread. Maybe your civilization tries to reduce the damage they do on their own planet by restricting their habitat to a minimum.
- They depend on a plant, which grows only in very specific conditions. Since you said your whole planet is habitable this plant shouldn't be food, but maybe a quite powerful drug. Withdrawal will drive you crazy or even kill you.
- Its just not worth it. It is hard to come up with reasons, why settling on habitable ground with very little resistance shouldn't be 'worth it', but maybe your planet has a lot of seismic activity and your people live on the only tectonic plate, that's spared of constant earthquakes. Your species might live in a different altitude (in a hole or on a mountain range like himalaya), so breathing on 'ground level' isn't really possible. Maybe your species isn't as advanced in any other sector as in space flight. They have no ships, no cars, maybe no medicine to handle all the different illnesses in other places.
I can come up with quite a few reasons why they didn't settle anywhere else, but it is quite hard to explain why they shouldn't have explored their planet at least to the same extend we have, when they are able to build sattelites.