When a group is mistreated, they are made to feel that they are inferior and should not take pride in their heritage. All "pride" movements have started as a response to this, to make oppressed peoples proud of their heritage. Black/Latino pride only exists as a response to the adversity those groups face.
Whites, as a group, have never experienced any adversity at all. Sure, the Irish have been discriminated against, as have the Jews (who are mostly white), but never have whites been oppressed simply for being white. That's why while Irish or Jewish pride is seen as acceptable, "White Pride" is not.
Whites already control most of the world, and have not have experienced any adversity as a group, so the only thing to be proud of is the fact that they're white. From this comes the white supremacist thinking. Which is bad, by the way.