Answer :

It may be because their bodies medication or something wrong so they need to live in a cool or warmer place or some times its just because! 
There are many reasons, you could probably group them into social, political, environmental and economic.

For example:

Social: their families / friends live near

Political: they may live in an area because they like the parties / people in charge

Environmental: they may like the landscape / weather

Economic: they live near where they work

These are only a few examples, of course, but I hope it helps! :)

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