Several years ago, the author said:
I have a couple of theories on why they didn't sell well; partly I suspect it was a case of being "ahead of my time" since Buffy and her ilk are doing well these days. Partly it was getting lost in the crowd of all the other horror writers who were flooding the market at the time. Partly it was that the books were not Classic Fantasy, which was where my market was building. For whatever reason, they did all right, but not spectacularly. I might try to revive the series some day.
There are indeed signs that stories of this type are popular now - Buffy, as mentioned, but also the TV series Charmed, Harry Potter, Twilight, and so forth. These aren't (except Potter, perhaps) "Classic Fantasy", but they are (especially the latter two) extraordinarily popular, and therefore selling well and making a great deal of money (which Lackey has repeatedly admitted is a major motivator).
If you read what she's said in the time between "The Last Straw" / "The Camel's Back" and now, there's a softening trend towards the stories - from a 'never again' attitude originally to a 'maybe'. It's likely that some of this is simply time - the ugly events are less recent and so less of a deterrent than they once were.
Given (a) a potential for better sales, piggybacking on the popularity of other series, (b) time healing the wounds of the nasty incidents, and (c) that there was always meant to be more books in the series (she's said that there was meant to be at least a forth, continuing on from Jinx High), new stories don't seem so far-fetched.
It's possible that fan behaviour has also improved in general and at conventions in particular, but I don't have any knowledge (either way) of that. Perhaps also the Internet has provided the "lunatics" (her word) with an outlet they didn't have at the time, and so they rant somewhere you can ignore them, rather than attack her directly.