I think it has a lot to do with all the shotgun only hunting regs in alot of states. If it weren't for that, rifled barrels and sabot slugs probably never would have gotten off the ground either. Because folks are limited to shotguns for jobs many want a rifle for, shotgun manufacturers are offering models that have more rifle like capabilities.
I have handled an 870 with a laminated thumbhole stock and I found it surprisingly comfortable. I think it would make a very nice peice for someone who does most of their hunting from a tree stand or blind. I'm surprised that they're not offering them on their slug guns, as opposed to just the turkey guns. Its not for clays, or dove, but it would serve a purpose.
Another benefit to the thumbhole stock is it allows you to get a thumb around the stock for a firmer grip when shooting heavy loads. With traditional stocks alot of folks have to let their thumb ride off to the side or it gets mashed into their face.