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.
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.
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.