Roar Off!!! by Master852 on DeviantArt (original) (raw)

Earth: "I'm stronger than you!!"

Legendary: "I have a better movie!!"

To answer your question, it is probably due to confusion from general audience goers. While strong fans like us can both comprehend and enjoy the idea of two Godzilla's fighting, the general movie goer will be confused. Godzilla is a singular being to a lot of them. The idea of Godzilla vs Godzilla is the same as Batman vs Batman to them. Awesome in practice and narrative, but not practical at the box office.