Good question ! I think it would involve
A wish to be able to fly
Any long term rule not to fligh, or not to travel in space, will be challenged.
People who have discovered engineering and technology also do science. A scientist will want to fly, to see if it can be done, to see what's up there. Many people want to fly.. on Earth, we apes had this wish, for centuries. Then.. you want to fly really high. You get into orbit. Or to the moon. To see what's up there.
Atmosphere
One practical hurdle may have to do with atmospheric layers. When your planet has a very thick atmosphere, it may be dangerous to travel through the turbulence.
Gravity
One other practical hurdle I can think of was found as answer #2 that is large gravitational pull, a big planet, or a relatively weak biology. It would be difficult for gellyfish to withstand the G-forces involved in leaving a planet.
Size/weight
I can add: when your planet inhabitants are technologically advanced and 2 inch tall, this will be a big hurdle to actually leave the planet. They may be able to fly, but building space-safe air tight constructs, as well as "powering up" for escape velocity may be out of reach.
.. or religious constraints
Your planet has a conservative clergy. One of the rules of faith is: you can't fly, only God can fly and the heaven is God's domain and you can't enter it, because if you do, you will fall and die.