Betta Fish: Despite my best efforts, my first betta fish died

I had him for a week, it took him three days from when he started acting weird to die. He showed no signs of stress, the only thing that could’ve hinted at him being sick was a lack of appetite, but I thought I just didn’t get him the right foods, so I kept trying to feed him different things and I’d seen him hunt detritus worms.

I got him from a local pet store, and I still want to keep bettas and give them good lives, do all bettas just have genetic problems that cause them to die randomly like this, or did I get unlucky the first time?

I didn’t even get to name him yet.