From a Tipler book:

“The unit of time, the second (s), was originally defined in terms of the rotation of the earth and was equal to (1/60)(1/60)(1./24) of the mean solar day. The second is now defined in terms of a characteristic frequency associated with the cesium atom…The second is defined so that the frequency of light from a certain transition in cesium is 9,192,631,770 cycles per second….”

Why cesium? And why that particular transition?