Why do stores keep Betta fish in small containers?

I'm not sure that they should be kept in a small container for their entire life, but I know that Bettas should not be with other fish, especially other Bettas. I once had a 20 gallon aquarium with an assortment of fresh water fish. Tetras were one of them that were in my tank. I put my betta fish in with the others and he would always try to nip at the other fish. I think they like to be in larger bodies of water, but it is best to keep them separate if you can.