*sighs* A lot of people ask this question...
There are a few reasons why. 1st, they are a cold water fish and Bettas are tropical fish. BIG difference lol! And (as Luimeril said) they need heavy
filtration that sucks your Betta in! D:! And, within a few years, they could eat your Betta...
If you want to get a compatible fish with your Bettas, make sure they don't have long, flowing fins like Bettas or are brightly colored. In a Betta's mind...Color=Competition.
Good tankmates for Bettas could be: Snails, Cory Cats, Ghost Shrimp, Small Tetras, 1 or more Cheery Barbs and White Clouds.
Kin are bad choices. Gouramis are cousins of Bettas. So do not put them together! Make sure you don't mix kin with Bettas!
Hope this helped!