Any company selling Ultraviolet movies has to obtain the rights to sell those movies digitally. Nook, being a relatively new player to UV, doesn't have as large a library as Vudu and Flixster. And even between those two companies there are some movies that are incompatable with each other.
There are always new releases and older films being added all the time so the list keeps growing. It is still comparably small, but compared to Nook Video's launch last year it is already much much bigger. My only gripe is that the TV show selection isn't growing much at all