It is indeed an evacuation driven constraint; a longer MLG would raise the wing above the maximum allowed height for non-slide assisted evacuation. Fitting slides would be a major and costly exercise, which Boeing does not wish to entertain. They've even gone to the length of inventing the fancy MLG on the -10, which will extend only at rotatation, then revert to its "folded" position when retracted and remain in that position until the next rotation. All to keep runway requirements below what would otherwise have been, well, a very large and indigestible number.
Internal Airstairs can either be extended or, if that's not possible, discontinued as an optional extra. Passenger bridges are height adjustable (otherwise a 737 Max wouldn't fit a 737NG bridge), as are mobile passenger stairs. A folding mechanism can be engineered, allowing for a longer gear without creating interference in the gear bays. But you can't increase the body height over ground without major revisions, eventually resulting in the loss of a common type rating.