Seconds are measured by the frequency emission of cesium. Why is a frequency from the emission spectrum of cesium used as the standard in defining a second? Why particularly cesium?
Rather than write something unintelligible, I'll quote from a page on cesium clocks.
According to quantum theory, atoms can only exist in certain discrete ("quantized") energy states depending on what orbits about their nuclei are occupied by their electrons. Different transitions are possible; those in question refer to a change in the electron and nuclear spin ("hyperfine") energy level of the lowest set of orbits called the "ground state." Cesium is the best choice of atom for such a measurement because all of its 55 electrons but the outermost are confined to orbits in stable shells of electromagnetic force. Thus, the outermost electron is not disturbed much by the others.
The point is that we want an extremely spectrally "pure" source so there's mimimal uncertainty about the wavelength.