Why aren't mobile lounges more popular?
I've used one once, at Dulles. There was no walking, no steps, escalators, or lifts. Just wait in a proper lounge, walk onto a giant bus sort of thing on the level, notice it drops to ground level, drive across the airfield, get lifted to the height of the a/c doors and board.
Compare with using Gatwick pier 6 - about 10 minutes of walking, several escalators, some claustrophobic corridors, the odd ramp. No fun with even a small amount of luggage, let alone with a couple of toddlers or an old person in tow. Or Heathrow's T5B, T5C - replace some of the walking with waiting for an underground train.
I suppose some people appreciate a brisk walk immediately on getting off a plane, and others are bursting for a toilet, but still I would think the vast majority of passengers would be better served by the mobile lounge than either by traditional buses or piers and airbridges.