I don't think there's anything unusual here. Critics and marketers alike gravitate to where audiences are, and generally speaking they aren't on Xbox right now. We saw a similar flight (in the opposite direction) in the PS3/360 era when the default disc became 360 discs
Call of Duty: Modern Warfare 4 has plenty of reviews. But look specifically for Xbox reviews and things get weird. buff.ly/yXTsSfN