There have been a bunch of rumors leaking out of Intel on the reasons. The biggest one was apparently that the Contact Over Active Gate just didn't work. That was used extensively in the GPU section of the die but not the CPU part which is why the 10nm chips released had working CPUs but no graphics. They've cut that feature from the process for the new version of 10nm which should help.

The other big rumored problem they were having was with cobalt interconnects and I have no idea if they've made it work or abandoned it or what.