The main reason seems to be that the taller roof is not structurally sound enough for the penthouse
The next reason is that the taller any extra weight gets, the stability of the van gets worse.
A final reason given is that the taller vans get narrower as they go up and that the bed would be a very narrow fit for the high roof sprinter.
I have a sprinter with penthouse and the bed is fairly narrow as it is.
The system seems to work for me because when I'm out of bed (and parked) I can raise the bed up to the ceiling and have plenty of head room. I only put the bed into position when i sleep so headroom is not an issue unless the top is down.
I think they fit a full/double width bed up there, but it's shorter than a typical mattress (made up for by extra headroom where the bed meets the roof). Good for two if you know each other well, otherwise one.
I have a PH top, but am envious of the upper cabinets everyone else has in their high roof. Depends on what you need...