Lotta good points in this thread, many of which I was prepared to talk about in great length. But, imma still throw in my take 'cuz why not.
SCS released ATS with so much less versus ETS2 at release. Way smaller truck selection, an environment with little to grab people's attention and a lengthy time before more content would eventually be added.
I'm not gonna whine and say the devs were lazy, the SCS team probably had some pretty crazy ideas. But of course, complications can and will arise, many of which can and will impact the game's final state, so I trust it was within SCS' best interest to release ATS in a playable and enjoyable state.
Overall, the game had a rough start and thus impacted its perception by a loyal base that most likely has a Euro-centric bias with ETS2, and couldn't be bothered to adjust to the nuances and unfamiliar nature of the American trucking world brought to bear in the ATS experience.
With all that being said, TMP at the time was already in full swing developing and improving the ETS2 online experience the team had made known to the world, and of course, when ATS hit, they were prepared to do the same with this title.
However, with the previously mentioned issues, the TMP team probably had difficulty trying to foster player retention in a modified game that at its base, was very lackluster to what ETS2MP was offering at that same period. There was some attention brought to ATS and ATSMP, mainly from a U.S-centric playerbase who, like the European crowd and ETS2, had implicit bias towards the American trucking experience. Still, even with continuing support from American players, the lack of base game content ATS had meant the European folk were not incentivized to purchase the initial map DLCs, which also lead to many just outright not touching the game, and finally, has lead to the current day situation of little to no community appreciation or attention to American Truck Simulator's TMP iteration.
But, as the saying goes.. It is what it is.