Humidity

Oceans form a big portion of the equatorial area and solar irradiance cause to huge amounts of evapotranspiration over these oceans, in other hand because warm weather has more humidity capacity, thus humidity level in equatorial air is high.

As air above the equator rises, it cools and is unable to hold all of the moisture--so it rains at the equator, creating the climate and moisture conducive for tropical rainforests.