Mud seems to play an unusually important part in many 'celebrations.' Memphis, a nearby 'city,' the Capitol of Bar-B-Que, has an annual Bar-B-Que contest each May. I'd have to check, but I think that may be the rainiest month of the year for this region...
It may be that local planners, at least in the northern hemisphere, know that the mud will make re-seeding the land easier.
Plus, a good coating of mud stops most pesky personal insects; even with their minuscule brains, they know to stay away from mud!