Because why not put this in the film? The Rowdyruff Boys could get significant development, aside from them reforming at the end of the film. Brick should notably get it the most, as I think the reason he reforms could be because of him getting jealous of Jared. As a result, he starts to gain a secret crush on Blossom, and therefore would like to end his threat once and for all. However, Boomer and Butch don't really get their development significantly. The only reason they could reform could simply be because they were tired of being dragged into schemes. And things prove to be the last straw for them in the climax when the villain reveals to them that (s)he was just using them as mere pawns for his/her scheme.