Most of my reasons have already been listed.
- Lack of Interest in the game.
- Another game came out at the same time and the other game was better, consequently the first game gets pushed to the bottom of the pile of "To Complete" games.
- No time to play the game, and when I finally do have time I've lost interest.
- Tedium, if a part of the game is very tedious and annoying, this can range from grinding to just annoying gameplay elements like searching for a small item in a massive map, then I might just give up.
- The game is terrible... Enough said.
I suppose one could say I have a bit of a ADHD when it comes to gaming.