Likely because there's no major consumer devices shipping them (or at least high-end versions of them) that could help them hit a scale that brings the cost down, which makes them less viable for a general public consumer standpoint, which means there's less time spent optimizing it.

I remember a wihle back when Google was shopping around for Intel replacements (likely a negotiation tactic), people were saying they should buy the POWER division from IBM (IIRC). That would have been really interesting...