The two primary mechanisms I am aware of are drastically simpler instruction decoding and lack of backwards compatibility. Both of these save on power and area

The two primary mechanisms I am aware of are drastically simpler instruction decoding and lack of backwards compatibility. Both of these save on power and area, allowing those budgets to be spent elsewhere. Often times on wider instruction decode, extra execution units, better branch prediction, etc. These budgets are much tighter on a mobile chip, but are still relevant on desktop design.