I think the answer to OP's question is mostly timing. A lot of people here with a full blown SMB had it long before many of the other vehicles listed were available, or readily available here in the U.S.
Ford Transit - New to us Amuricans, unibody design, not many (and no cheap/readily available) 4wd conversion parts.
Mercedes Sprinter - Still fairly new here, no aftermarket 4wd conversions worth the trouble. Many old SMBer's are switchingto these though. Not as much dealer network or mechanics who can work on them but that is changing. The Winnebago Revel seems to have sold a million or so units already if any drive through a ski resort parking lot is a telltale.
Mercedes Unimog - Hard to come by here, still very rare even within the already small world of offrroad campers/camper vans.
Iveco Daily 4x4- Same as above tenfold
Toyota Landcrusier with camper conversion - Same as above, pretty much got to import any of the good conversions. That is changing too. Also, very small for most people who have had a van.
VW Syncro - Not much availability, dismal reliability, unfathomable prices on the #instavanlife market. You can get a helluva lot more van for half the price people are asking for these things and they aren't made anymore so there is no softening of the market in sight.
More and more people in the U.S. are going the way of heavily modified E/F series Ford or other heavy pickup chassis cab builds using Total Composites boxes, repurposed or home built camper shells, utility bodies, flatbeds with 4Wheel Camper type setups, etc. These are essentially the domestic versions of the Mogs/Ivecos in Europe and Australia. If it's one thing that we have at the ready in the U.S. it's heavy duty trucks to build from.