The CPUs in these consoles are exceptionally weak, developers can't do a proper multi-threaded code yet, and the GPUs in both are weaker than a 7870. The Xbox One is more like a 7850 and the RAM on the One is DDR3 so that slows it down further.
Ubisoft claims it is Microsoft and Sony telling them to water down the PC version and make everything 30fps (also check my sig for their other BS claim). Either way, it's all ****ing stupid. We shouldn't suffer because we're on PC. I honestly don't want to buy an Xbox One but my love for Halo 2 rules over all and I know those ****ers at Microsoft won't release it for PC.