MP Soap contains glycerin. Glycerin is a humectant which means that it will naturally attract moisture in the air to itself. Humid climates experience the issue the most.
In saying this, there is such a thing as too much sweating. Using the microwave to melt your soap often causes excessive sweating because it distrupts the water balance in the soap. Adding too much fragrance or additional oils can be another reason. If all else fails, perhaps try another brand of soap base.