Everyone has their opinions on why the Sprinter/Promaster/Transit is more appropriate for camper van conversion over the other. Here’s mine.
Not sure why the van community aspires to Sprinters. While Mercedes is known for their luxury cars, the Sprinter (at least NCV3 and earlier) is certainly not a luxury product. It is as about utilitarian as a plastic fork, and while you can excuse some traits of a vehicle as a tradeoff by having good characteristics in other places, I would assume that priority #1 for a van would be reliability and serviceability, and this criteria alone should put it at the bottom of most people’s lists.
What I mean is, while people might buy a Mercedes product as a luxury car knowing full well it may have issues down the road, the dealer can loan you a vehicle while they work on yours. They can’t however, loan you a small apartment, and nobody has a fully built out backup camper van.