I don't know for sure, but my guess would be that it is because the ToT races take place late at night, and so there's no pressure to get runners out of the way. The other events all happen in the morning, and so Disney needs to have stricter pacing requirements so that runners aren't tying up the roads and parks all day. That is really the purpose of the pacing requirements, not to penalize slower runners/walkers, but simply to ensure that the race is completed on time to impact other guests as little as possible.