If you shut down and start over around the new idea, maybe you’ll be able to raise money around that idea, but then again, maybe you won’t. Nothing’s guaranteed; and if you find you can’t raise the money again, you end up with nothing at all.
Meanwhile, in the existing business, there’s a big pot of money already raised just sitting there for you to use. Using it could mean problems down the road, of course. But not using it means problems right now, so it’s not hard to see the appeal of just grabbing it and figuring you’ll cross those bridges if and when you ever actually come to them.