well, probably because they (the BBs) don't exist yet...and probably because when they do, there won't be very many ISIS splined cranks for track-specific use. also, probably because for high-end cartridge BBs, most people look to phil, because most track-cranks, and especially the NJS-approved suginos which seem to be the new hotness right now come in square taper. but i honestly bet it has more to do with the CK BBs not existing in reality (yet).

but really, any ISIS MTB crank can be used for single speed use, and actually, any ISIS road crank can be used for fixed gear use. but there's only one track-specific ISIS crank that i know of (FSA vigorelli carbon).