I think Whip nailed it for a lot of them--what used to be fun wound up being work.
Some found that they wanted/needed to make more money.
I've heard of some getting sick from exotic woods.
I figure some just burn out.
Some just never figure out how to be competitive--you can't spend 40 hours on one bow and expect to make a living--unless you have a customer base willing to pay really big money. According to one bowyer I talked to, if you have 40 hours in one bow it's because: you are counting drying time for glue/finish/etc.; you are doing some very intricate customizing (inlays, checkering, splices, accents, etc.); you don't have the equipment needed; or you just don't have the knowledge/experience to do it any faster.
Overhead costs can eat you alive. Advertising, catalogs, brochures, t-shirts, etc. all add up quick. I've heard a few estimates that with some companies around 1/3 of a bow's cost goes straight into advertising--that's $300-$400 plus with some. So much for getting what you pay for. I've looked into it, and don't doubt that estimate. That's why I don't have even a brochure, and if I ever get around to getting shirts done I'll sell them rather than give them away. Ain't nothing free--somebody has to pay for it somewhere.
Some may come out with something "new" that keeps them working overtime for a while, until someone else comes up with an improved version/new material/etc.--gimmicks sell, but generally only for a short time before a new gimmick comes along.
Seems to me it's like any other small business--some make it, some don't. It's not always the ones with the best product that make it, or the ones with the lower quality that fails.
Chad