The best reason to say why Intel changes sockets so often is because they rolled most of the chipset's functions into the CPU starting with LGA1156. This means that if they want to change their platform to any significant degree, they have to change sockets in order to accommodate the necessary I/O coming from the CPU.
Intel sockets used to just supply power and the FSB to the CPU. Unless they needed more power or significantly changed the FSB, they didn't need to change the socket. That's why LGA775 stayed around so long- they didn't change the bus nor did they change the amount of power the chips used. AMD is much the same way- their non-APU sockets supply I/O lines to memory and a HyperTransport link or links to the chipset/other...