Because #blackpeopletwitter was a thing before Reddit picked it up. Often, various content with that hashtag would make it to trending, and thus many more users would find it.

The Black People Twitter subreddit gained a lot of popularity, but then the mods felt a need to ensure that actual black people were commenting, and they made a verification system. You would have a specific label that allowed you unfettered access to the blackpeopletwitter subreddit.

Occasionally, the subreddit would be locked to only certain flaired users (aka country club threads). This made it easier to actually hear the voices of the very community, especially because they were often posts about violence toward black people. This all made sense.

The White People Twitter subreddit didn't make sense. Like many things IRL, white people took from black people, revamped, and made their own thing.