Why is cosmic background radiation a microwave?
The cosmic background radiation has cooled down to microwaves.
Soon after the big bang, the universe was very hot and filled with high energy gamma rays. The universe was much smaller then.
As the universe expanded, the background radiation got progressively red shifted. This has resulted in all of the background gamma rays from the early universe getting longer wavelengths and reduced energy.
Now, the universe has expanded so much that the background radiation is in the microwave region of the spectrum.