I'm not sure if you know what a CPU Socket is. The socket is where you plug in a CPU and contains a certain number of "sockets" for CPU pins to connect to. For example, socket 775 CPUs have 775 pins.
Adding Cache is often done to more expensive models of a single CPU line, all of which use one socket. However, when you add more cores or make it faster, you must be able to transfer more data, which means more pins, which means a new CPU socket.
"If at first you don't succeed; call it version 1.0"