I think the reason is because there is anti-LGBTQIA+ sentiment in many populations around the globe. Hell, you see it here on this very forum. SE being a profit driven company doesn't want to risk alienating too many paying customers, and that unfortunately includes the bigots. So from their standpoint the best action would be no action. As one person said, a social media post is probably the best one could hope for.
If it makes you feel any better, I'd never expect there to be a Black History Month seasonal event either.