Heavy sweat can be weathered through (no pun intended) in a no-gi mma match. But if the body was continually being doused with rain, it'd be like participating in a mud wrestling contest. Footing becomes a problem too in a sport where leverage is key. That's also why mma and even boxing do not allow excessive dousing of water or application of grease on fighters by their cornermen in between rounds. They need to be towel dried first as grapplers can't get a strong grip and boxing gloves could glance off their faces.