No, we didn't miss the question. We think that it is deliberate and policy-driven because WDW does not WANT to put a those buses next to the MK, not because they don't have room. (They could easily make that bus plaza larger if they wanted to, but I don't think that they want to. Looking at it via Google Earth, I easily see spaces for 5 more stands on the current path, plus there is room to use some backfill to open up another whole line of stands.) I do not ever recall being able to board a bus in front of MK to go to any other park, only to resorts.
Walt did not want ANY vehicles driving up to the front door of the MK. An exception was made for resort transport after the 80's resort expansions increased resort traffic to the point that it might have arguably have overwhelmed the TTC's tram system, but that doesn't mean that the company has given up on the idea at the MK. None of the other icons is as striking as the Castle, and it still has impact when seen from a distance. From a practical standpoint, now that hopping isn't on every ticket, it just makes more sense to put those buses at the TTC where they can be used both by guests leaving the MK, *and* guests arriving via off-site buses and private vehicles that are being parked there for logistical reasons.