[mtizim]

It's not, we just use NAND everywhere because they're easier to make with transistors. You can get functional completeness with a NOR instead, or alternatively with some different combinations of other logical operators.

https://en.m.wikipedia.org/wiki/Functional_completeness

We even implement AND gates with NANDs in electronics (because they're way simpler), but we might not have to limit ourselves to a single base gate with mechanical computers.