FPGAtoASIC is faster and easier to develop, gets you out before the competition, but costs more per chip.
Just my suspicion. Not proof.
I'm sure I read somewhere a while back that the KNC solution is an FPGA conversion, and that would fit with it's power consumption. Going to 20nm at this time is a mistake - the process isn't fully 'productionised' yet and might take another 6 months before yields are stable. the NRE is truly horrendous and same for the wafer costs. You always pay a premium for the newest process.
As a result, newer 28nm designs will easily beat their 20nm one in cost efficiency but since they've already sold a ton of them, I doubt very much that they'll care. Their customers might though.