White isn't capitalized because white people tend to self-identify with other ethnic groups (Italian, Irish*, German, etc.) There is no "White" culture per se, outside of the historical revisionist beliefs of white supremacists.

Black is a culture, and capitalized as such, because the descendants of the African slave diaspora brought to the US were denied the ability to identify with their country or ethnicity in the same way as white people, so the only identity they could form after getting their freedom was one based on their collective racial identity. American society wouldn't allow them to be anything other than "Black" so "Black" is what they became.

* although interestingly, the Irish weren't always considered "white." Race is a weird social construct.