The element gun, as it shoots things that cover the four elements, would be somewhat unusual and, as James Gunn points out, that would need to be dealt with. Nobody else has a gun like this, we haven’t seen it anywhere in the Marvel Cinematic Universe before, and that would need to be explained. Since every movie has to do its share of explanation for its characters and universe, the gun just adds more that would need to be explained, and the director wanted to make his story as streamlined as possible, so things that didn’t need to be there, like the element gun, were dropped.
