To use stars for navigation, you need to be able to see the stars (or planets). The availability of stars at helicopter altitude in say western Europe would make it considerably less reliable and available than GPS.
Typical astronav accuracy is also MUCH less accurate than GPS.
(Prior to long distance RNAV), for the floaty boys astronav was OK in mid-ocean because an error of 5, 10 or more miles did not matter mid-ocean. And it did not matter too much if you took a couple of days to get a fix in the middle of the Atlantic. If a ship could hit something, they would stick a light on it (those were the days). Astro wouldn't really be good enough for any helicopter ops