I actually boarded a plane via a mobile lounge at Dulles Airport in Virginia in the early 1990s. Now, they apparently only use them to shuttle passengers between the main and satellite terminals.
Planes got bigger than capacity of mobile lounge x number of doors on plane.
Another problem they’d have with mobile lounges today is that based on the one time I was in one, I don’t think they’d have the room for all of the carry-on baggage people seem compelled to bring on planes these days.
And load carrying capacity for the morbidly obese. Scissor lift mechanisms don’t do well with off-center loads.
I notice that the new generation mobile lounges, called “Plane Mate”, in use at Dulles and Montréal airport, have replaced the scissor lifts with screw jacks on the corners. The “fins” that project from the top house the jack screws.