It has been a while since I worked in the laser cooling field. My recollection is that atoms with a single valence electron have a simple energy level scheme that is more conducive to using lasers for controlling their motion (i.e. slowing and trapping the atoms). So basically, hydrogen and the alkali metals.
Rubidium was a good choice in the late 1980's and early 1990's because the transition at 780 nm coincided with diode lasers that were readily available. Not sure what the current state of the art is regarding other alkalis' wavelengths and the easy availability of lasers today.