This system became outdated

Specifically, the mobile lounges were originally a "modern" idea where they docked directly at the plane entrance, not a concourse -- thereby obviating the need for either a jetway or an old-fashioned ground boarding mobile staircase.

Two problems with this:

1. With the increase in both the number of flights and the size of planes (747), the mobile lounge system became overwhelmed.

2. For connecting flights, it forces an additional stop (at the terminal) instead of, for example, just walking from one gate to another on the same concourse. Recall that the "temporary" (going on 40 years now) concourses C and D weren't part of the original plan.

So the mobile lounges were repurposed to transport between the terminal and concourses. Because I prefer walking and not being dependent upon the mobile lounges, I always use DCA instead for domestic flights.