I think this is mostly for weight savings; the direct mount hub eliminates a separate hub with the drive pins, pressure plate and wheel nut. The hub itself can be made lighter and simpler because there's no need for a machined register and mounting face. The mounting bolts basically replace bolts that were already in the rim flange so there's no appreciable weight gain in that area. My guess is the weight is more important in most cases than being able to change the wheel quickly.