http://wccftech.com/intel-coffee-lake-lga-1151-pin-configuration-detailed/

Intel confirmed in their official Coffee Lake briefing that the new processors use a vastly different pin configuration compared to previous generation processors, and as such, there is no backward compatibility with 200 or 100 series motherboards. While Intel did mention it, they didn’t go in to much detail but today, David Schor (Engineer and Industry Analyst in the field of Semiconductor & computer architecture) has revealed that the Coffee Lake processors are indeed using a different electrical pin configuration, confirming Intel’s claims.

According to David, the reason we don’t have Coffee Lake processors compatible with older series motherboards that feature the LGA 1151 socket is the change in pins. For instance, if the pin config changes on a processor, the sockets on the motherboard need to be configured as such. It’s not a process that can be done via software as its more of a hardware level change.