Can't speak for everybody but after cod 4 came out Halo just stopped being fun to me. Reach was the last halo I bought and I played that for like a month and then went back to cod. I don't know what make a game fun but halo lost that aspect. Halo 3 was the last fun Halo game.........
http://imageshack.com/a/img801/6426/f7pc.gif
^Yes that's me ripping it up in the GIF. :)