It basically comes down to two factors:
1. Goblins don't die of old age.
2. They can live anywhere on land.
Creatures can only settle where they have START_BIOME, EXCLUSIVE_START_BIOME, or SETTLEMENT_BIOME. BIOME_SUPPORT lets them travel over biomes, but doesn't let them settle in them.
Elves are immortal, but their biomes are extremely restricted, being able to settle only in forests, and can't even claim territory outside of them, so they usually won't even spread beyond the particular forest they start in.
Dwarves are a little less restricted in where they can settle, but they still can't travel across swamps, deserts, or oceans.
Humans are pretty good explorers, but they can only settle on plains. Their movement through deserts and wetlands is also pretty slow.
Goblins, on the other hand, have ANY_LAND for their start biome and biome support, which means their civilization is basically a green-skinned murderous fungus. They can spread anywhere without any restrictions or movement penalties (except over water) and settle anywhere they spread. They'll tend to be most thick in the swamps and deserts since nobody else wants to live there, but they'll pretty much be found anywhere that anyone else isn't. Add that to their negligible death rate (outside of war), their propensity towards conquest, and their demon leaders, and you can see why the goblins usually end up inheriting the world.