On a 747...

1L and 2L are available. 3L is over wing so not convenient. 4L is possible but is slightly difficult to manourvre equipment on to due to wing proximity so best left alone. 5L is a little dangerous to open due to curvature of cabin and is essentially emergency exit.

1R and 2R are available (but often being used by catering during tight turnaround). 3R is over wing so not convenient. 4R is possible but is slightly difficult to manourvre equipment on to so best left alone. 5R is a little dangerous to open due to curvature of cabin and is essentially emergency exit.

The ramp is a dangerous area so far better to keep folk entering through 1L and 2L where they can be supervised. If you board through say 1L, 4R and 5L you need a lot more supervision, buses etc. If using jet bridges, you still need the supervision and a very complicated and, no doubt, expensive infrastructure.

I am sure some cleverer folk than I have analysed this and come to the conclusion that it is just not worth it. I do actually regularly fly a carrier that boards a 777-300 on remote stands from 1L and 5L and it causes havoc with gridlock in the aisles. It could be better managed but I see little benefit even if the airline was able to achieve better boarding management...

On smaller aircraft on remote stands, I can see the argument...