I think the idea behind glofish bettas is that they can fill the same niche as normal bettas, meaning small tanks aimed at kids and newbies. Other glofish require regular tanks with all the effort and money that entails, plus they're usually schooling fish. Around here, a single glofish costs $6-8 dollars, and they're supposed to be kept in a school of 6+, so you're looking at $36-48 just for one school of fish without the tank or anything. I bet you'll be able to get a betta and special glofish betta tank for $50. Lower cost = more sales.
Also, a lot of people buy bettas without planning to just because they feel sorry for them in their cups, whereas sick fish normally put off customers. Plus, if a normal betta dies, people now have a reason to get a whole bunch of special supplies for their new glowing betta instead of just putting a new fish in the old tank. Again, more sales.
Additionally, pet stores have limited shelf space, so the special glofish products eat into the profits of competing companies, giving any company selling glofish stuff an edge. Really, it doesn't make sense to not
make glofish bettas. Unless, of course, it turns out that customers prefer the normal ones.
I don't understand the appeal of GMO fish when natural fish or even fish selectively bred are already so attractive.
That's like saying you don't understand the appeal of tetras when goldfish are already so attractive. Variety is the spice of life. Genetic engineering is a tool, just like selective breeding (which isn't all that natural when eggs or seeds are irradiated to promote mutation), so what's wrong with using it?