hmmm, perhaps this appeal isn't as strongly felt by the general public as cute birds slamming into pigs?
Is this contrary to expectations? This is what we mean when we say, over and over again, that "ideas" don't matter a hill of beans. To misquote the best line of the movie: if they had had the idea for Angry Birds then they would have developed Angry Birds.
Anyway, Crush the Castle is itself a clone of Castle Clout, but had _far_ greater success than its predecessor, even though they were visually similar (nearly identical, actually).
The reason I think that AB is more popular is it fits a more general audience, the graphics/story/etc are all neutralized so they're covering every demographic while CTC is more male-oriented.
Although Angry Birds has undoubtedly made more cold hard cash the CTC franchise (3 titles) would have plays in the 100s of millions by now.
That transcends considerations like "times played" or even "money made".
Unique users in the 100s of millions? I'm doubtful.
When a Flash game is very popular 1000s of other websites copy the swf file and host it themselves exposing their own audience to that game - and CTC is 3 very popular titles.