Why isn't two-door/split boarding on commercial airplanes more common?

Split boarding or two-door boarding sounds at least to me like a no-brainer. Basically you open both the front and back doors and let passengers board from ends of the airplane. Seems at least to me it's a lot more common with the terminals that use air stairs that you need to walk across the apron to get to rather than jet-bridges, as it's pretty easy to just roll two air stairs up to the aircraft.

Why isn't this more common? Boarding and deboarding a plane is slow and very prone to a single person holding up the entire process as there is no room to go past them in the aisle. Allowing boarding from both the front and back doors will at least half the time it takes, and especially with deboarding, gives passengers two options for exits which means a single person can't hold up the entire plane. If the people in front are being slow, just leave from the back.

I know that designing a jet-bridge that can line up with the back door is pretty difficult especially since you have to fit it alongside the jetbridge for the front door, but why not just use the jetbridge for the front door and roll air stairs up to the back door and have half the passengers go down to the ground and walk across the apron? I'll gladly spend a few minutes walking through the heat or rain if it means we can board and deboard in half the time, especially if it means we don't lose our takeoff slot from a slow boarding process and have to wait on the tarmac for even longer.

What do you think? Are there practical issues that this is not done more often? Or is it simply because the airlines don't really want to pay for more gate services?