I would expect there is some legacy aspect to it, although Intel building the Itanium (no legacy restrictions) just made a box that used more watts per instruction than their 32 bit line and wasn't that much better off. That said, things like the 'half carry' flag, segment pointers, and a large power hungry FPU don't help the Intel story.

I expect the primary difference one will find is that Intel has more "large" cores in their chips (full fledged cores) and the comparable instructions have some "gotchas" (like the flag mention) which make them harder to retire early in the pipeline. Toshiba made a funny comment at one of the Microprocessor forums that for their microprocessors, if you only compare the instructions that both they and Intel implement, the Toshiba processor is better in every way.

What is clearly true though is that compatibility aside, ARM chips are becoming serious contenders in the desktop/laptop space sooner than I expected them too.