Alongside what @Flaming. has said with licensing (keep in mind with trucks its not even one license per-brand most of the time, its per model and even if TruckersMP wanted to license the older models such as the old VNL they probably couldn't), the files for players who don't own ATS and were expecting to see them on ETS2 would have to download at LEAST another 700-1000 MB of data (for just the truck models, not even tuning parts, paintjobs or trailers) which they already don't have the legal permission to distribute themselves.
SCS has in the past removed mods that port ETS2's trucks to ATS and ATS's trucks to ETS2 from the steam workshop and their forums because they don't want to get in trouble with the companies, it wouldn't set a very good look for TruckersMP.
The reason TruckersMP is able to use the trucks as they are now is because its ONLY a layer on top of the pre-existing game, which doesn't require licenses from the truck brands. Mods that involve truck brands the mod makers themselves haven't licensed are absolutely infringing on trademarks and copyrighted stuff (especially the paid ones), they just skirt a fine line of either not being noticed or not being popular enough for the brands to care.
Of course, I and most players would love for this idea to come to fruition but unfortunately its far outside the scope of TruckersMP and probably even outside the scope of SCS to pull it off