allow me
there are two answers, the very simple one first-
Beane, for all his missteps, managed to pluck Allen from out of the morass of Darnolds and Rosens. He's responsible for the single most important Bills draft pick ever and that's always going to trump whatever roster deficiencies may exist in a given year. McDermott has no such singular victory. In fact quite the opposite- he's directly accountable for one of the worst collapses in league history.
the more complex answer is that NFL roster construction is as we know a bicameral process executed by the GM and informed by the tendencies and preferences of the staff. there is very little doubt in my mind that Beane's drafts and pro personnel moves would differ significantly under a different HC. that, along w the obvious fact that despite incredibly disingenuous protestations to the contrary we very much did have Super Bowl caliber rosters in both 2021 and 2022, is why McDermott is correctly under more scrutiny than Beane