There weren't any legitimate technical reasons for switching from LGA1156 to LGA1155, but Haswell and LGA1150 are introducing major changes to power delivery and voltage regulation. Given that power delivery accounts for something like half the pin count of a modern CPU, a new socket for Haswell is quite justified. (Although, those power delivery changes only really benefit the mobile market, but Intel's long been unwilling to produce different dies for the desktop market and the mainstream mobile market.)