I do like the idea and putting them together is a really good idea. And overall they do look pretty good. But if I have to take a guess, it would have to be because of how the shading looks and how some of the poses look. Using Voltman as an example, I'm comparing him to Bombman.
Notice how Bombman's chest is shaded. Compare it to how Voltman's is. It could be shaded to be closer to the style than how you did it. Secondly look at how the foot looks in Volt's jump. Looks kind of weird when you compare it to Bomb's jump. Fixing mistakes like these would definitely improve your chances to submit this.
EDIT: Here's my attempt at fixing it up.
I'm not the best at shading so it definitely can be done better. But you do notice it at least looks cleaner.
Notice how Bombman's chest is shaded. Compare it to how Voltman's is. It could be shaded to be closer to the style than how you did it. Secondly look at how the foot looks in Volt's jump. Looks kind of weird when you compare it to Bomb's jump. Fixing mistakes like these would definitely improve your chances to submit this.
EDIT: Here's my attempt at fixing it up.
I'm not the best at shading so it definitely can be done better. But you do notice it at least looks cleaner.