Because it is something called the water cycle. When it gets hot, the body of water (let's use a lake for example) evaporates, because of the sun's heat. Then it travels to the sky, clings together, and becomes a cloud(s). Next, when the clouds get too full, they rain. If it rained up, then we'd never get our water back, and it would ruin most of our tourist attractions and natural resources. The water cycle is an amazing thing, so never take it for granted. (I said you in general; not anyone in particular. Just everyone)..
YO ESPERO QUE ESTE AYUDAS!