I'm not entirely sure what you're trying to do. Are you picking on TDU modders for leaving?
People do this as a hobby, they want to make a car and they do so. They share it with the community so they can make people like us happy.
It is a LOT of work to do that and the work itself is not fun, I can assure you that. The end result is the main driving force.
When a modder has gotten bored of a game and doesn't have the motivation to do a mod for it, why should he do that?
Also, keep in mind that TDU has very few active people anymore. And if you put on 50 or 100 hours of work into a mod and put it online just to get a single reply, it is underwhelming. You are very proud of the work you've done and you want to see that people have enjoyed that and that it was worth investing those hours. If you don't get anything in return, you will lose interest in doing the work as well.
GTA San Andreas on the other hand has a huge player base to this day. If you were to make a good mod for GTA SA, you'd get thousands of downloads and hundreds of comments. Therefore, you'd be very happy yourself, because you know you've made so many other people happy.