Sad but true, that in most cases each cycle on the CPU leads to improvements that make the new CPU incompatible with the older boards.
Intel runs what they call a Tick, Tock plan currently were the first version of the CPU (TICK say nehielem) gets refreshed about a year later with a mostly compatible update (TOCK Westmere) (sorry am more familiar with servers then desktops so don't know those off the top of my head)
Sometime extra CPU sku for speed bumps / steppings are also released during this phase
The Tick / Tock is basicly to address the problem with your needing a new mother board every generation.
The older familys are usually around for a couple more years after the new family releases, but new development is focus on the newer family.
One nice thing about this plan, is the price of the last generation processors is usually much less than the current generation.