Are we talking Disney musical, other types of musical, or just musical films period?
As far as the comic book costumes go... well, I think it's the difference in the medium. One is artwork, the other is fashion. Costume designs for superheroes in comic books needs to be something that works for a two dimensional page.
Not to mention that artists will design costumes to look good against the backgrounds of their pages.
On the other hand, films have a much more complex array of colors in the background, being real life, and when it comes to costumes, the simplicity of them (or sometimes the complexity of them) will make them stand out like a sore thumb. I guess.
Also superheroes are tailored to the costume in comic books, but in movies, the costume has to be tailored to the actor.
Of course, I'm no expert, I'm just throwing out a bunch of guesses based off of potential reasoning.