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human
[Star Wars] Why do Wookies and Twi'leks still mostly live on their home planets?

Wookies and Twi'leks have been part of the republic for thousands of years. Since 4000 BBY at least, based on my knowledge of KOTOR. On earth, Europe has only been aware of the Americas for 500 years, and the americas already have a larger population than Europe, with just 5% of that population being indigenous. If Wookies and Twi'leks have been immigrating through the galaxy for at least 8 times as long, why are their home planets still their major population centers? Wookies are understandable, with a lifestyle dependent on a more specialised environment, but Twi'leks can live anywhere humans can, and they're not nearly as common as humans, while Ryloth is nearly all Twi'lek in population. Sure, you can find a Twi'lek in any seedy bar in the outer rim, but why don't there seem to be any major population centers of Twi'leks elsewhere? Same with the wookies, why is Kashyyk the only Wookie planet worth mentioning 4000 years after Revan and the Exile travelled the stars with two Wookies in their crew? Why is there almost no immigration in 4000 years?
gpt
Low population numbers and cultural resistance. Wookies have a sizable population and a fairly active trading network in their sector of the Galaxy. However they don't have a population in the high figures that would make kashyyk feel overpopulated or unable to sustain them. Their culture is also one of family that remain close throughout their lives. They just don't have the desire or need to migrate and integrate into the galaxy beyond a few honoring life debts or those taken by slavers and the like. 

With Twiileks it's a little different. Ryloth is a pretty brutal planet that is barely habitable to them so they really don't have large populations anywhere. They have a very strict clan structure that has many clans completely killed off for the transgressions of a single member. These clans are highly restrictive with many indebted or enslaved to their clans leaders for most of their life. While many Twiileks have left ryloth it's not a massive migration figure. Most Twiileks found off ryloth are descendants of former slaves taken by different outer rim groups like the Hutts and Zygerians, or lone exiles that fled to avoid a worse fate. Some are sponsored by their clan for different reasons but many that are found off ryloth have no ties to the planet or it's clans anymore. Twiileks have a long history of being enslaved and brutalized by other races and their own kind so they have a strong stigma against leaving their homeworld unless necessary.

To add in as well Twiileks and wookies aren't compatible with humans or near humans for reproduction. While some races flourished through interbreeding on colonies many others simply can't out breed humans to become a majority or peer race on most colonies.