My opinion probably isn't to valid as I left AH 2+ years ago, but for me the reasons really came down to a couple of things:
-Gameplay: it wasn't horrible and there was always something to do, but it was usually the same thing to do. Add in the good fights (for me at least) were becoming harder to come by.
-Updates: No so much graphics, eye candy is nice, but mainly the updates in content. Since leaving AH I've moved onto iRacing, while the game type is different, but the part I enjoy the most is how fast content is added. Even at 15.00 per track and car, plus my subscription I gladly pay it as I know every 13weeks there will be a new track or car added. Perhaps someday AH could look at how iRacing does laser scanning and decrease content build time.
Those are just my thoughts on why I left...might be the same as others, might not be.