Why are tropical rainforests cleared?

The most common reason is commercial (and often illegal) logging. Timber, especially tropical hardwood, is valuable and you can get rich cutting down the trees and selling the timber.

The other reason is slash and burn agriculture. Indigenous people clear a section of forest and burn the trees and undergrowth. The ash provides a small amount of fertile soil which allows them to grow crops. When the soil loses its fertility, they move on and clear another area.