[simias]

Given that I mostly work on embedded software I'm not the best person to ask but I suppose that if you're writing software that uses heavy performance optimizations going from one CPU model to an other is not necessarily trivial.

On top of that most server farms wouldn't completely ditch their old computers to replace them in one go, so you end up with two different vendors to deal with, potentially two versions of your software etc... That increases the maintenance burden quite significantly.