It wasn't sustainable. Maintaining enough of a calorie deficit to lose that much weight made me depressed and made it hard to do my job - I think better with more calorie consumption. So I stopped dieting for much the same reason other people stop smoking - it was having negative effects I didn't like.

(Also, the system I was following (hacker's diet) incorrectly claimed after losing the weight you wouldn't gain it back from simply returning to prior eating habits. Because it didn't account for the now well-known metabolism-lowering effect of dieting.)