I think in part because there is direct legislature that makes certain hiring practices illegal and cause for retaliation (obviously this is fraught with problems in proving intent like everything, but if I'm from a particular ethnicity and I have a track record of dismissing candidates that are able to be isolated down to a protected trait rather than ability to do the job, then I can see that as being relatively obvious.) but Like i said it's fraught with external factors, but as far as I know a caste isn't considered a protected class (probably because it doesn't have strict parallels in the West).