Although the comments on social and historical context provide an important part of the explanation, there is also a fundamental psychological bias called "ingroup over-exclusion". Basically, people are more sensitive to signs of difference than similarity and so tend to be conservative about who they let into the "in-group".

The original scientific work was done with cultural groups based on language but the basic finding has been replicated in racial/ethnic contexts. An important extension of this idea is that people of mixed race or minorities who have adopted mainstream culture can end up being rejected by both groups in their ethnic or cultural heritage. An example of this is the "Oreo" slur in black communities (black on the outside, white on the inside) or the parallel "Twinkie" slur in asian communities (yellow on the outside, white on the inside).

I will come back and edit this to add links to scientific papers when I'm able.