Pluto is a lot brighter than the stars at that distance, so they are underexposed (to the point of invisibility). If you look at pictures when New Horizons was

Pluto is a lot brighter than the stars at that distance, so they are underexposed (to the point of invisibility). If you look at pictures when New Horizons was farther away and Pluto therefor a lot dimmer, you'll see plenty of stars.