IMHO x86 has better existing facilities for different operating modes and virtualisation --- going back to the 286 protected mode descriptor types --- that make it a good fit for being the "host" of an ARM or other architecture. Also, the x86/PC boot process is far better understood and open than those of the widely differing ARM SoCs.
I feel there are a lot of folks in MS that understand how important it is to have a consistent boot process. They tried to standardize this with the boot system of Windows RT by having it map closely to that of BIOS. I think they just wanted to have ARM run like x86 by having a standard interface so they aren't building multiple different binaries for different machines.
Of course this was basically bolting a good idea to an cement block dropped in the ocean, it didn't help that they had locked keys on secure boot so that users couldn't fiddle with this.
How do you figure that the x86 is more open? I can download and build all the code for an ARM system right now. There are even standards docs for most of the individual pieces.
I can't do the same with any modern x86 systems and the one company trying (purisim) had to do some pretty serious reverse engineering of Intel's blobs to get there.
