Okay say Suicune is the best Pokemon in the tier. Then we can atleast agree on the fact that Mega Aero is the most common Pokemon in the tier, a fact proved by both the ladder and SPL. This means that if you use Mega Pidgeot, then for your opponent, checking it is as easy as simply slapping Mega Aero on their team and calling it a day. And Mega Aero can actually be slapped on pretty much any team as a check to everything on offense, as one of the best sweepers in the tier, as a stall-breaker, and as a hard check to Pidgeot all in one. There is pretty much no reason not to use Mega Aero, which only makes life more difficult for Pidgeot. Aero being the most common Pokemon in the tier is more than enough reason to keep Mega Pidgeot out of S.
Sure Aero loses to Suicune, but you also have to take into account that losing to Suicune does not affect Aero's viability because it is also used for other things, specifically the roles I stated above of being the best check to offense along with being an amazing stall-breaker. Pidgeot's viability specifically depends on its ability to sweep, which is massively hindered by the prevalence of Aero.
Not to mention Aero can actually break past Suicune with a little prior damage if Suicune switches into Hone Claws:
+1 252 Atk Mega Aerodactyl Stone Edge vs. 252 HP / 252+ Def Suicune: 165-195 (40.8 - 48.2%) -- 7.4% chance to 2HKO after Stealth Rock and Leftovers recovery
Pidgeot on the other hand has absolutely no chance of breaking through Mega Aero without significant confusion hax as Hurricane only 3HKOs even after Stealth Rock while Aero always outspeeds.
Another thing to consider is that using Pidgeot takes up a Mega slot, which means you are forgoing the use of Mega Aero. This puts further restriction on your own teambuilding meaning you need to take up team slots to check a lot of threats, which you could actually simply do by just using Aero lol. I'll say it again, there's pretty much no reason to not use Aero right now, and its prevalence is what keeps Pidgeot in A+.