We outnumber others, either by out-breeding them at some point in time or by out-killing them. Use one or combine at your leisure.
The source can be technological or biological.
Hard edge, by outkilling the others:
Humans were the first who discovered how to smelt iron. While the others were still using bronze armor and swords, here came humans with iron swords and shields and armor. It was absolute slaughter. Humans got advantage by either killing or enslaving other races.
Humans were the first race who discovered guns.
Humans are absolute psychopaths when it comes to battle for survival: other races give up after some damage, we fight to the bitter end. And not only that: kill one of us in our village, we band together and kill your whole village.
Ye may kill for yourselves, and your mates, and your cubs as they need, and ye can; But kill not for pleasure of killing, and seven
times never kill Man!
Rudyard Kipling, The Jungle Book
Humans are extremely territorial as well. We don't run from dangerous animals, we band together, get our spears then go out and kill them. And keep killing them, to keep our territory free from other predators and what we perceive as harm.
Basically, for every single dangerous animal you can think off, there was at least one person somewhere thinking long and hard "I wonder how I can kill it. A spear? An arrow through the eye? A knife? A trap? Everything to remove that gator/lion/dragon from the water source."
or "It is venomous, I wonder what can I do if it bites me? Cut off my hand? Make antivenom? Say my prayers and tell others that it is venomous and don't touch it and to kill it on sight with a large spear/ huge stick?" etc.
Soft edge, outbreeding and outsurviving :
Humans were first who started industrial revolution / found out how to have their children reliably survive to adulthood. We found out how to purify water (by boiling it in order to make beer and giving resultant beer to children, instead of dirty water), we found out how to have our women and children survive childbirth, we found out how to keep our kids alive (Hint: children are more fragile and susceptible to cold than adults, thanks to losing heat faster because of their smaller stature. Who knew?! We did!) until they reach adulthood. We found vaccination against common diseases. The vaccines don't work for other races.
We found out how actual medicine works for our species. The others go to witch doctors. We invented dentists. The others go to blacksmiths to pull their infected teeth out with tweezers.
Humans were the first to find out how to agriculture. Optional: The plants we decided to cultivate are mildly to horribly poisonous to other races but are fine to us. Or just taste bad for them. You don't want those dirty goblins and elves stealing from you, right?! So don't grow things they can eat on your field!
Which leads to displacement: If you burn a forest that has food which elves and dwarfs can eat and replace it with fields with food they can't eat, what are they going to do?
That's basic habitat destruction, that is.
Due to glitch in biology, other races find us extremely sexy /authoritative/scary on instinctual level. It's a supernormal stimulus.. Our females are like super models for other races. Our males look scary like bears or sound like gods to them. Or something like that.
They find our children absolutely adorable and cute. In fact, our babies are like cuckoos for them: so adorable they forget to take care of their own children all so they can care for ours. Again, see supernormal stimulus. Our babies crying sounds a lot like their babies crying (similar like cat meowing sometimes sounds like baby crying), but the opposite is not true.
Or maybe they find our adults absolutely adorable and cute? For example, because goblin child has smooth face and large eyes and sometimes furry cheeks which adult goblins lack. But human adults have in abundance!
Other races can breed with human females but the resulting offspring is always fertile genetically human female. Other races can't interbreed with anyone but humans.
Human males breeding with females of other races leads to for (original race) sterile half-orcs, half-elves etc. Unless the resultant half-human offspring breeds with human females, which has 50% chance of getting fully human offspring and 50% chance of miscarriage. If half-human tries to breed with non-human, the result is always 100% miscarriage. Basically, difference between mule and hinny.
Humans are true omnivores and can eat all sorts of food that is poisonous for other races. In fact, they can eat halfling food, orc food, elf food, goblin food etc. Goblins can eat orc food and halfling food but elf food is extremely poisonous to them. And so on. Which lead to us having edge and living in environments that others find extremely inhospitable. Basically, elves live somewhere, orcs live in some parts, halflings live in some parts but only humans live everywhere.
We are talking poisonous like theobromine is poisonous to dogs. It is found in cocoa and dark chocolate. Humans love chocolate! Dwarfs and goblins and halflings die from a single lick of it. Now imagine that weakness for something not as rare, like for example grain and flour and corn. Or maybe we are the only race that can drink and process animal milk into adulthood. The others get violently ill and/or die if they try.And milk and flour are used in basically all human recipes.
Also, relevant video (Vorta are immune to most forms of poisons from DS9)
Similarly, we can survive much greater range of temperatures than other races. Orcs, dwarves and elves overheat quicker, halflings and goblins freeze quicker than us. Making it worse, we developed materials which allows us to build housing in places other races find extremely inhospitable.
Basically, win hard enough against others and that gives you all the breeding edge you will ever need.
The reason humans didn't kill the others?
We still need somebody to clean the toilets, carry heavy stuff, work in the mines, clean the chimneys and make toys for our children.