Intel kept changing the spec and not telling anyone, so they got little support from the case manufacturers. BTX concentrated on cooling the CPU, when there are now loads of other things that need cooling in a case too. I think I read a few months back that Intel had given up on BTX.
Personally, I just don't think it offered much over ATX, especially given the huge improvement in cases. You don't need BTX to get all the benefits - you just need one of today's decent cases, and you get better overall cooling, and you get to use a standard ATX motherboard.