I'll share what DID make me leave since I did leave for 5 months before returning 6 seasons ago.
The reason I left was the engine not rewarding me for my hard work. I do not know what was going on but the update I left on the devs decided to add an upset factor to help smaller, noob free-to-play teams, have a chance against pay to win teams or experienced free to play teams that manufactured a quality team (me). It would run the simulation in their favor giving your team random red cards and unusual performances such as your ST missing goals after dribbling around the GKer. So basically I got to a point where I was tired of watching ads for free rests, farming free packs, training, and staying up late to attend matches just to lose when the other manager isn't even there and their team is 30% less than me. And all the Assistant manager would say was "the difference is they took their chances" or "tough day" or my favorite "we just couldn't upset the favorites." ...The favorites. I had to quit.