Why only put focus on that specific time?

It's part of their disordered (not the best term, but bear with me) thinking. Everything is black and white for them. Thus, there must be a single, pivotal start to something, before which there was not that thing, after which there is that thing. For gay rights, that is Stonewall(tm) the First Ever Time(tm) that LGB people fought back. It's the singular point before which was "bad times" and after which are "good times" - full "right-side-of-history"/Whig history bullshit. And thus by inserting the TQ+ into it, it makes them important to that pivotal, critical, formative moment in their thinking.

I'm not sure why they do this, or what causes this specific flawed thinking, but it's a common trend I've seen in a few places.