I think a part of it is down to demographics with disposable income. Teenagers have a taste of freedom and some pocket money and the next gimmick films is a good way to spend it. It's the same as they mature into 20 somethings. In their 30's they may be more career focused and have less time, a good chunk of them will tire of novelty and move towards more interesting/arthouse films. When kids come into the picture there's even less time and money so things change again, then the cycle repeats.

So, at least from my opinion, "new" will always be a good sales tactic to catch attention.