Because you're more or less starting over. The technical work you do would carry over, to a large extent, at a new company with a clean cap table. All the "real" work is customer discovery and early GTM. Your investors signed on for an earlier company, that company didn't pan out, they're adults, they get that, they don't get a claim on your next company.

It's not like this is an iron rule, but it's something you should be thinking about. If the only things in common between your new direction and your old one is your cofounder and some of your tech stack, you should have a good reason to keep the investors on, right?