It's a confluence of factors. LGBTQIA+ is sort of a gender/sexuallity/ phenotype physicality solidarity alliance and the actual boundries has grown in scope since the 80's.

Like take for instance asexual people. Asexuallity became a part of the solidarity when people reached out over the internet and and started realizing that there were a lot of people who just don't feel sexual attraction and that there are certain widely accepted forms of social coercion that revolve around pushing people towards sexual attraction. But asexuallity as a part of the LGBTQIA only really became a thing in the early 2000's. Non-binary trans identities are much the same. A lot of people were feeling the way they did about themselves in isolation but they had no frame of reference to think that they were not just the odd person out.

The other half is a society wide re-examination of compulsory heterosexuallity/cis gender hegemony. There are way more people out there who no longer define themselves by who they've chosen to have physical sexual experience with and now a lot more people are more frank about defining themselves by the range of people they are attracted to. Like if the majority of people artificially penalize a bi-person for choosing a same sex relationship a lot of people will just take the easier path and just narrow their choices or keep their liasons with the restricted choice secret and not assume the label.

I before I came out as trans initially figured I didn't count as trans because I both wasn't physically transitioning and my industry is somewhat hostile to trans people so I was very closeted ao I figured the label only really belonged to the people brave enough to live out of the closet... But eventually someone found me and was like "No, it's not aspirational. Even deep in the closet you are still trans."

This combination of destigmatization, solidarity messaging, the inclusion of whole other groups (like intersex people, gender minorities, asexuals) broadening the scope and outreach to the closeted means that more people generally self identify as LGBTQIA or queer.

Animal kingdom wise we're still less observably sexual fluid than other primates. Bisexuality is actually pretty ubiquitous particularly amongst male primates with it actually being the overwhelming norm in some species so chances are we are probably actually haven't seen the curve level off from suppressive stigma.