Very simple. Winning is fun, losing is not. The easiest thing to do is blame the coach. It is the coach's job to win basketball games first, and the fun is a two way street. If that mindset is in place, the snowball is forming and rolling down the hill. Most parents place a higher priority on playing time than winning and coaches are the EXACT opposite. Not a good marriage!
Playing on the varsity level requires respect for the coach, but the coach IS NOT the reason you play or not play. You play for the passion of the game, period. A coach should never determine whether or not to play. If thats the reason being given for not playing, then the passion isn't there and there is probably another reason for quitting. The easiest thing for a HS kid to tell his parents is that "I don't like the coach". "It's not fun anymore". Unfortunately there are parents who buy into those excuses (especially when times are tough in losing situations). What lesson does that teach?
Winning is a habit, unfortunately so is losing.