As far as I'm aware this is just a decision made by each tier's leaders and UU has always operated under the 51% majority rule. There might be another reason like a generally lower playerbase resulting in fewer voters (and therefore a simple majority making more sense).
Hogg would probably be more qualified to answer this than I am though.
Edit:
This thread has more info, but basically the TLDR is that lower tiers are inherently more liable to fluctuation because they get drops and OU doesn't, so the 51% majority is in there to make sure it's reasonable for them to make changes.
So true
I guess I can see the merit but I'm not really sure why I'd ever use this on Nasty Plot sets since +2 Psychic/Sludge Wave sends both Grasses flying and Thunderbolt nukes Scizor, and on pivot sets I guess it's nice to ohko Scizor bu it doesn't really do enough to the Grasses to really be worth considering. Maybe it's cool on super specific teams but in general it seems kind of outclassed.