There is no in-universe explanation - at least in the series.
However, I'm not sure they did mention it in their reports:
ODO: I just read the report that you wrote and I wanted to thank you.
GARAK: Me? For what?
ODO: For not mentioning my desire to return to my people.
GARAK: I consider the entire conversation as something best forgotten.
ODO: As do I. Quark has expressed an interest in renting this space if you're not going to be using it.
It's not clear how much "the entire conversation" consists of.
But even if they did, mere knowledge that the technology existed didn't mean that it could be reproduced. The device was, apparently, of secret Obsidian Order make, and whatever knowledge needed to create it was presumably lost in the ensuing battle with most of the Obsidian Order. We've seen several instances within TNG, VOY, and DS9 where people went to extreme lengths to steal technology via espionage, rather than just recreating it. I think it's entirely probable that the Federation did try to recreate the technology, but was unsuccessful.
But it may not have been highly prioritized. The technology only prevented shapeshifting - it may not have been considered tactically important, because you can't possibly cover everything within the anti-shapeshifting field. And even if you could, for all we know, there's health hazards to non-shapeshifters.
assuming they could be made either small enough to conceal or powerful enough to cover a wider area.
This is actually a very big assumption, one that generally only holds when it is necessary for plot reasons. ;)
The Federation did go on to develop more lethal anti-shapeshifting technology, at any rate. Perhaps they did use some of the insights from the Obsidian Order technology, if they were able to obtain any information on it. We don't know. But the Federation anti-shapeshifting was much, much more strategically important, so ultimately, the anti-shapeshifting field wasn't important.