Only gamefreak knows, really.
however, I'd rather say that the mechanics changed instead of calling it a "nerf". first of all, getting the crits is much easier now although the reward decreased:
+0 1/16 (6.25%)
> +1 1/8 (12.5%)
+2 1/2 (50%)
+3 and above Always (100%)
opposed to gen 2-5 and eariler's
+0 1/16 (6.25%)
+1 1/8 (12.5%)
+2 1/4 (25%)
+3 1/3 (33.3%)
+4 and above 1/2 (50%)
or even worse: gen 1's
BaseSpeed / 512
this means Pokemons like an absol with super luck holding a scope lens using night slash or a farfetch'd with a stick using slash has a 100% chance of critting.
moves with 100% chance of criting is also better now as their BP increased to 60 instead of 40 meaning its technically 90 instead of 80 as it was before. Pokemon with sniper is still insanely powerful now if not much better as their chance of critting can be 100% if they use focus energy and is holding a scope lens