[Behind the Blue Wall]

It’s a subset of the bigger question of why they don’t board planes for the rear.

[Blue Jays]

I am with you and totally see what you are saying.
If they had a way to "back-in" an airplane and put the jetway to the door at the rear of the fuselage, I'm sure they would.
Maybe it has something to do with wing clearance near the terminals, other aircraft, along with the jetway itself.