Have thought of 2 other reasons. One is the arrival of kids as TS4 toddlers are a serious pain in the caboose, so that particular household soon becomes more aggravating than it's worth. Fortunately, I tend to play rotationally so go on to another household not so afflicted.
The other is outside wrecking. About a year ago on the almost fully loaded installation, either the game or Microsoft did something to both the saves and tray folders so that all the files in them have an X at the head of the file name. I could replace the tray folder thanks to the one from my other installation on an older machine, but the games are toast. Had to start from scratch. A shame as there were some interesting games in there.