Yeah, Japan has a wider range of vocal songs for Transformers. The Anglophone Beast Wars and Beast Machines themes were instrumental (the BM theme wasn't even written for Transfomers but taken off a Leftfield album) and virtually every other Anglophone TF theme is just some version of the G1 "More Than Meets The Eye" theme. So if you're gonna run a comp where a lot of people are gonna be singing say G1 MTMTE, RiD MTMTE, Armada MTMTE, Animated MTMTE etc. then it'd get pretty bland fast. Whereas the Japanese themes are all unique and distinct from each other. I know there's the language barrier thing, but honestly whenever I go to anime conventions - the majority of people singing Japanese anime themes don't speak Japanese but they're able to sing the theme songs.

Otherwise the judge(s) might be stuck listening to people's renditions of MTMTE and Linkin Park all day long!

Although thanks to movie Bumblebee there's a host of other songs that have appeared in Transformers...