Question

Rajasthan is a desert why

Answer 1

The whole of Rajasthan isn't a desert, there is some part of it which is a desert, the remaining is inhabited by people. 
Anyways...answering your question, the temperatures over there make Rajasthan hot, non humid, and dry, upto the levels where part of it has turned into a desert. The climate, the altitude, the distance from the seas and oceans, all are factors that decide the factors of Rajasthan being a desert.

Hope it helps
^_^