the problem is the audio hardware drivers. most manufacturers dont write linux drivers, and they dont release the necessary specs for linux groups to write the drivers.
take the M-Audio Audiophile for example. yes, it CAN work under linux(after much work) but the s/pdif I/O will NOT work.period.
as far as higher-end interfaces, RME is the only company I know of that has full working linux drivers, but they only work with Ardour which is still a "alpha" application.
things are getting better now that ALSA is officially part of the kernel, but it still is going to require time and manufacturer support to get the proper drivers written