Those numbers are based on how often those resistances appear in the MM, not on how often those creatures are typically encountered. The typical adventurer fights lots of undead, and a lot of undead are at least resistant to necrotic damage. And many of the monsters with cold resistance are rarely encountered.
Certainly psychic, radiant and force damage are good, but so is Thunder, and (people forget this) magical s/p/b.
Yeah my group defaults to thunder with things like chromatic orb.
Undead not that common and even if they are it's dead easy to use radiance as a cleric anyway (or just turn them).
Undead you encounter them at some point probably but getting hit with say fire.