Seems to me like updating the sockets isn't all that necessary considering that the pin count doesn't seem to change much at all. My guess is that Intel is again trying to maximize profits by pushing people into the new hardware, not unlike the methods that Apple uses to keep the sheeple coming back. Anyway, new socket unveiled.
http://wccftech.com/biostar-unveils-skylake-ready-gaming-z170x4-lga-1151-socket-motherboard/
The difference between AM3 and AM3+ is one contact on the socket. The Difference between FM2 and FM2+ is two contacts. It's not just Intel.
They change the socket so people who haven't done their research don't end up putting the wrong CPU in the wrong motherboard. They don't have to change the socket with "ticks" because the architecture is just shrunk and subtly tweaked from the previous generation, so it is more or less compatible with the last chipset. It matters with "tocks" because the architecture of the CPU changes more dramatically, rendering old motherboards unusable.
If you do not update the socket with the chipset, you are going to run into a whole host of incompatibility issues that you never have to deal if you do change the socket. You will have people putting CPUs into old MOBOs and wondering why x,y,z doesn't work, well because not supported. You will have the opposite as well.
It is good practice to rule out the old to usher in the new with this type of hardware. You don't have to upgrade every year or two like some people do, CPUs are not increasing enough year on year to actually make it worth while, so by the time you go to upgrade, unless you like to throw money at hardware, you will be on the newest best platform and won't be worried about holding onto any old hardware that would have been worthless either way.
I'm a little miffed you already forgot about LGA 1156. It's only been 6 years.