I was just wondering if there was any particular reason that runs must be done in English? It's not really a big deal, I'm just curious.

Yes. Pokemon speedrun must use English. Just a rule.

I understand that it's a rule, I was just wondering if there was a specific reason for that rule

Probably just for ease of majority.

I think it's because people trying to watch and learn the speedrun need to read the text in order to understand what's happening, and English is the most widely spoken language among the community. Personally I know I wouldn't watch runs in Japanese because I would understand none of what they were doing.