AW: because there isn't one yet. The IOP342 detailed here [link] isn't 3x faster on single core performance (what counts on RISC OS barring miracles) and neither is any ARM11 based chip. Its getting closer, but all the time x86 performance is increasing at a far faster rate, and hence the speed of emulation.
If Castle brought out a brand new ARM based machine, and the Virtual Acorn stand set up next door, lining up a whole range of Windows and Mac desktop and laptop machines running VRPC faster for less money, how many do you think Castle will sell? There's a few people still stupid enough to buy one, I'm probably one of them, but it would never make any money and finish off Castle for good.
For native hardware to make sense, we need the same sort of leap in performance that the StrongARM offered over the ARM7. There is a very tiny chance that could happen if low power x86 cores start seriously eroding ARMs core market, but I don't think its a realistic possibility that either x86 will do this or ARM partners would respond in that way.