I can answer this easy;
A. There's lots of different ethnicities of white people, and most white people can trace back their cultural heritage, meaning there's nothing stopping you from celebrating your history. Irish people were persecuted at one point as well, and they got St. Patrick's Day to celebrate Irish history (not that there's much to actually celebrate there, but America sure seems to love it.)
Black people, on the other hand, had their ancestors brought over in boats from all over africa, and the families were separated, meaning there are very, very few Black people that know where they came from. Most of their history starts in America itself, and most of it involves overcoming cultural hurdles imposed on them BY America specifically because they are black. Black History month is kind of a blanket celebration for all the shit that happened over the years.
White people, in contrast, don't really need something like that, because white people don't have such a dramatic history revolving around the color of their skin.
B. What the fuck do you need a straight pride celebration for? Nobody ever lynched a straight guy for being straight (no, sjw whining doesn' count.) Being a hetero's just how most people are, and nobody ever cared about that, so there's nothing to really celebrate.