I left because I no longer had the time to fly 20-30 hours per week. Only logging on a couple of hours per week, I had no interest in spending 5-10 minutes flying to a fight that I no longer had the skills to win and would lose very quickly. I commute in real life, I don't want to do it for "fun."
Also, it appeared to me that HTC was hellbent on eliminating the enjoyment from the game. Remember tank town? Hundreds of people would spend hours jumping into a fight. Get killed, jump right back in. It was like eating popcorn and a helluva lot of fun. Then it went away. Now if I want to fight in a vehicle, I have to drive for 5 minutes to get killed by someone I can't see. Why bother?
Finally, HTC kept tweaking the rules, by the time I left I had no idea what it took for my country to win the war, and if we were winning, it was made so unpleasant that I'd log off rather than fly an old school bus armed with a .22 rifle that only fired on Tuesdays because my side had an advantage in numbers.
I miss my squadmates, but if I want to fly, I jump on IL-2, do a quick mission to get my fix and don't have to deal with folks who know better than me what I should be enjoying.