TruckersMP never said it will be cross-platform immediately. The game is completely different on Linux binary-wise, they will have to do the same thing (decompiling, porting, testing, bug fixing) again for each platform they support. ETS2 being native is a negative factor in that regard - TruckersMP should be native as well and support Linux native ecosystem in order for it to work. One cannot simply grab a Windows-native application and make it work against a Linux-native one without a hassle.
Another factor is a cost vs. impact. Right now, Steam counts Linux user base as 1.12%, even less than macOS users. The real count for ETS2/ATS is unknown, but it is very rare to see a person playing these games not on Windows, in my experience. We can estimate it as max. 50 people based on 4.688 concurrent player peak last week. It is not viable for 4.640 users to wait at least twice as long for an MP to update, just because those 50 people want to play on Linux. Sounds kinda harsh, but that is a crude reality. And do not forget about a cost associated with it. Game Devs will have to learn how to decompile/develop against a Linux-native app, this will require a substantial amount of time.
There are benefits of having a Linux port of TruckersMP, but they are minor compared to a number of issues which will be introduced with it. Maybe in the far future, when TruckersMP will reach a beta stage, it will be mature enough to spend the time on porting.