One reason I play Hattrick is because I like the cyclical nature of the game. You start with an idea about what kind of team you want to build, then you train long-term (up to 3 years in real life!). While you build, you can test and try to comprehend the aspect of the match simulation that you want to master. I also like the tactical choices you have to make for each match. But the most important component is the community.
When I started my studies back in 2002, one of my roommates played Hattrick. He was really excited when he explained the game to me, so I started playing. In the first year of playing, I got an HT-mail from someone who was living nearby. At this moment he is still one of my best friends. We both started playing again when we first stopped playing somewhere in 2008.
Why I stick around in Hattrick? I think it is love :) I mean, everything I can do to help Hattrick to stay as immersive as it is, is very important to me. I was a moderator, a translator, an editor - just because I felt that Hattrick, in a very peculiar way, was mine. Now I'm a National Coach because of the same reason. I think Hattrick is my game. I want to play forever and stay here with my friends.
The Hattrick community is colourful, it really helps expand the Hattrick world. You can find plenty of helpful managers that can help you with Hattrick and real life questions.
I would say the community is the strongest reason I have stayed in Hattrick for so long. The game too of course. After having trained my players and played in the French Division 2 for a while, I started focusing on training international players to help the Moroccan team. Today 80% of the defense of the Atlas Lions was trained by me!
What I like about the Hattrick community is that there are real people and you don't just compete against the computers. You play against each other, but you can also help each other in the forum and talk about everything.