Why evaporation is less in rainy season?

For any given temperature air can only dissolve so much water in it, the amount of water dissolved in air for any given temperature is called the "relative humidity" of the air.

Thus in the dry season it is likely that the relative humidity of the air will be low and the air will be able to absorb a lot of water, while during the rainy season the relative humidity will be high and the air will not be able to absorb more water very well.