Bigbruin.com has a posted part 2 of their CPU Socket Basics Part series

As CPU technology advances, the sockets that correspond to them are forced to change as well. The new physical sizes, internal architecture, and thermal properties require that the 'packaging' evolve. A negative side effect of this development is that motherboards wind up with limited 'upgrade paths' when it comes to adding a faster processor.

Bigbruin.com Article: CPU Socket Basics Part 2, AMD