I'm just started, really like what you have done with the massive player counts on your servers, very unique, I have found most people to be friendly or just busy making money in game which is cool.
I would say it's something you could do to bring more people, events or whatever.
The numbers sort of show that,
30K people playing ETS2, 10% on your servers.
5K people playing ATS, less than 1.5% on your servers.
So its not a ATS issue, Its not an issue at all IMO as any single group bringing 10% of a large player base to 3rd party servers is a big achievement in itself and really not that common.
My first time trying to join the ATS servers, I thought you needed all the DLC and ProMods to join, that could be a ME thing tho. But it is there at the bottom saying promods and a error / popup when you don't have it, I just went back to ETS2 servers, just putting that out there, not saying its the reason just something to add other than "More Events".