Simple answer: this was first game I EVER played, and I put too much effort on it.
It gives me so much nostalgia about old days. I can't leave this game, it means a lot to me. I spent 7 years playing this game, and I just don't want to stop. I put my soul into farming&upgrading&getting better items and etc. I can't let myself forget about this game... this time, joy of getting good items, joy(not) of getting five epics from premium packs... this emotions of getting LMs... Ah such a good feeling. This game changed me a lot, and I can't let it waste.
You can't stop playing the game you wasted so much time on. You will always return, always check what changed, check new updates, and MAYBE attempt to come back and play it again. You can't forget about it; you can't forget all the good memories about this game. They always will be with you.
But let's look at the bad side. We all know that this game is dying; no new items, p2w offers, *cough* *cough* hackers, toxic community... if this wasn't first game I ever played, I would probably leave it because it's literally unplayable; It has bad graphics, it's unfair(SM and fair are parallel lines), and it's luck-based. Everything is based around RNG which makes it even worse. So yeah, my answer probably will be "nostalgia's sake". Because without nostalgia, I wouldn't spend even one year on this game.