The real reason is that when they were focusing on ETS2, there was a growing demand for a big rig style US counterpart. The eventually released the ATS base game with CA, NV, and AZ but the scale of the states was way smaller than the community cared for. After sales and demand to rescale ATS increased they finally went ahead and did an overhaul of the base game and stated releasing DLC states as they moved east across the states.

Because of demand and the success of the rescaling, they are now developing the 2 games in tandum. A lot of US people like ETS2 and a lot of EU folks seem to like ATS. I’m stateside but absolutely love how Scania trucks look lol. I think it might be that we don’t have cabovers here anymore and maybe it’s a bit of nostalgia since my dad used to drive truck.