Because Rubidium is a secondary frequency clock. Meaning it is not as accurate and needs to be calibrated against a primary frequency standard or the Caesium atomic clock. Having said that, if you watched the video above and heard the examples of actual audio files with drastically exaggerated jitter, then you probably noticed that it is not as obvious as people may think. So I believe that comparing a Rubidium to a Caesium clock in audio would be similar to arguing which of the gnats that flew overhead, heading south, had a bigger set of balls.
But it is only a matter of time before the Caesium atomic clock chips are commercialized. DARPA has already made them a reality.