Why there aren't any pandemic diseases by prions?
I think that you could argue that some prion diseases do form epidemics or outbreaks. In some cases it does appear that people have referred to BSE/vCJD as pandemics. There is even some journalism on the topic, and see also here.
Infection by prion from the environment does occur at some rate, and it is true that it is these prions are the more transmissible. See Table 1 in that linked paper for more information; it appears that the discovery of prion excretion through e.g. feces is relatively recent.
For less transmissible variants it is true that you usually have to directly ingest a fair amount of prion from (i.e. eat) an infected individual to be affected, which is a harder means of transmission because it implies cannibalism (which is not that uncommon biologically, but still).