Well, with the Mihaljevic case, the curtain was found 300 yards away from her body along a rural road. That's a couple of football fields away. If she had been found wrapped in it and they still held the info back, then I would question the decision a lot more (don't get me wrong, like you I do question it as an outsider, but I understand I'm not privy to all the evidence). But if I understood correctly, forensic techniques didn't advance enough until 2021 when they were able to definitively say that her DNA was on that curtain.
We heard in the news articles about the tank top in Holly's case that it was found "in the vicinity" of her body and that only within the last six months was it referred for
further forensic testing (implying that some testing had already been done on it over the years), so I'm wondering if it was a similar thing to Amy's case where it was found in the vicinity of, but not right next to Holly's body, and until recently they did not have definitive forensic evidence that it was related. When I walk in the woods I do see abandoned clothing at times, it's not rare to see that, so there would be a need to determine pretty conclusively that the tank top was actually related to the crime. I am frustrated that it took this long too, but I don't think it was simply a matter of police not wanting to do extra work.
Source for the "vicinity" and "further forensic testing" comments -
In '93 case of Holly Piirainen, authorities say clothing could hold answers