Because whoever wins this game won't be at a big enough rest disadvantage already.
There's already a monday night game (Philly/Tampa).
It sucks, but I think, from a Buffalo POV, having a game with less whipping wind is probably a bit more of an advantage than 1 day less rest.
It probably doesn't matter for Pittsburgh, because they would be playing baltimore off the bye regardless, so one less day doesn't matter, they are already at a disadvantage.
If Buffalo wins, they can play one of Cleveland, Houston, or KC. All of whom play Saturday. Buffalo also doesn't have to travel, so that evens it a little as well.
There is a bit of disparity there. But, in the end, it's like playing after any other monday night game. The coaching staff will know their potential opponent on Sunday, so they can do some early game planning then. It's just one less rest day for the team if there are injuries.