Wait that's not it. The show switched to digital painting for Season 5. The show costs basically the same as any other show on the network. Its not an issue of money, Danny decided to move on for his own reasons since th show has been on for 10 years. Cartoon Network was just taking their sweet old time airing this. But yeah, not too expensive to make the show.
Pixmintro
Wasn't his fault it took a long time to air in the US, it's Cartoon Network's fault for being a bunch of fucktards.
JakBaronKing (Updated )
Whatever the reasons....Cartoon Network can't afford to make anymore episodes because Danny won't convert his style to computer animations.
He is very strict about the whole 'living-outline' effect. It's when you animate a character using only a pen, so it literally moves per frame.
It's really expensive to do that sort of animation. Especially for a 90 minute movie.
But in a sense, that's what makes his series pure GOLD