The other fish are too big for the betta to eat. He did chase them when they were first added but they were way too fast for him and he'd lose sight of them behind plants and stuff. So he eventually got bored with it. Now they all come to the top and eat together and he pays no attention. There's one baby pleco he might have been able to eat, but he never even noticed them being added; he only cares about stuff that swims around, not stick to glass.
The only living thing my betta ever ate was shrimp, which they're known to do. They were pretty big shrimp, and he ignored them for quite a long time, but then they molted and they were goners.