New sockets are usually driven by some new external technology - DDR3 to DDR4 for instance, or the need for more power/ground connections. While I can't say for certain, some changes are made just to physically prevent older parts from getting plugged in - usually for a good reason, like the voltages are different, etc - because even though a package says "this isn't supported" - if it fits, somebody will plug it in, and then claim no fault.
What I can say, is that the reason is never "oooh, oooh, here is a way we can screw the system builders and make them buy more parts, let's change the socket!"--
"We are half-hearted creatures, fooling about with drink and sex and ambition when infinite joy is offered us.... We are far too easily pleased." C.S. Lewis