how does something like this go undiscovered until now

I don't know the specifics of this particular insect's origin story, but for a surprising number of insects & arthropods, the answer can be summarized as "nobody bothered to look closely enough".

Sometimes that's a literal fact -- the critter is so well disguised, lives in a remote or hard to reach location, or is so uncommon that nobody's ever noticed it before. But with surprising frequency, it's a case where previously undiscovered species have been right under our noses (or feet or rocks in our suburban yards) this whole time and we simply did not realize it. It's not that we don't notice them, just that nobody's ever taken the time to really study them enough to spot the differences from one closely related species to the next so we simply assumed they're all the same.

For example, there are species of beetles that about the only reliable way to tell them apart is to count the hairs on their butt when they are larvae. As adults, they are nearly indistinguishable. Now imagine that nobody ever took the time to study larval butt hair.I guess what I'm saying is, we need more funding for bug butt hair and general bug butt hair awareness, because it's a thing.