Simply space. Even the small quiet Honda 1000 (900$)takes up a lot more space than an alternator. If my AC still worked I would obviously not remove it to add an alternator, but since it is basically just a pulley, I would like to remove the ecm voltage regulator from my house batteries, without lighting my CEL.
But in reality, with my solar and upgraded wiring, I do not need to add another alternator, just saying it could be done, and If I did not have solar it would be more imperative to do so to be able to get any life out of the batteries. Because the OEM charging circuit is insufficient for recharging a bank of deep cycle batteries. With the OEM wiring being the first weak link, followed by the ECM's voltage regulator, then the alternator itself.
What I am wondering is if Alternators for Vans with serpentine belts are wound differently, because the pulley is significantly smaller with a serpentine pulley, and thus must turn much faster, and probably charge a whole lot better at lower rpms than a 74 mm V belt pulley, unless the rotor and stator are wound differently to compensate for this.