the biggest problem has got to be the severe under-education of the consumer. not only do the pet stores keep bettas in bowls, bags, and cups; they also sell "betta kits" which consist of a little bowl or tiny plastic tank and some rocks or marbles and maybe a plastic plant. when the store offers these things as legitimate housing for the fish, the customer trusts that this is all they need to do to care for their new "decorative pet".
and of course when the fish just sits there because it doesn't have enough room to swim around, they get bored with it and either dispose of it or stop taking care of it. if these people ever got to see their betta in a large tank, they wouldn't think it was the same fish. i rescued my betta from my sister who had it in a big enough tank, but was not caring for it properly. he's currently living in my 55-gallon planted tank with 10 debauwi catfish and has never looked better.
if you keep a dog in a little cage, you can get sent to jail for animal cruelty... but a fish isn't important enough to deserve the same protection? that's messed up.