Is there any particular reason for why nobody has ever submitted a run on the English version? Or has it just not been done yet?

Yeah, just no one's done it; the text is probably a little slower but I'd be up for having a separate English leaderboard

I'll be doing runs when I get around to it and I'll be on English to know what I'm reading. If I think I've got good enough muscle memory and watched enough runs to know what basic things are, I'll get a JP copy, but only if it saves significant enough time.