Well, first off, we DONT know that he hasnt mind controlled someone close by. Last second sleeper agent attacks are a fairly common trope in this sort of story. But, even if he doesnt do that, there is a good excuse. He doesnt want to take that risk. If any of the people there suddenly start acting even a little different, it could raise suspicions.
Combine that with him needing the time and opportunity to cast his mind control, and its a dicey prospect. That being said, literally anyone who has been away from the crowd for more than a minute could be turned into the last second sleeper agent. Luna might be a sleeper, but then again, she is just so great that it would only cause one strip drama as she breaks through out of twu wub.