It's probably because NX is like a souped up PS3/Wii U in terms of horsepower. That's basically what the Tegra X1 is.
Developers are going to have to decide whether or not they want to do the work at that low of a power envelope.
Some devs will say yes, others will say no.
From the sounds of it, NX is something like double a PS3/360 in raw power probably with a lot more RAM and a much more modern architecture.
So PS4/XB1 ports are probably possible if assets/resolution are scaled back (same way Far Cry 4 and MGSV are able to run on PS3/360).







