Yeah, Lackey's appeal really only makes any sense when you consider that she was filling a vacuum for a lot of people. She does a lot of boneheaded-to-offensive stuff, and if there is a list of ways that you can be a bad writer, she has clearly carefully studied that list, and put checkmarks next to most of it, and then added a couple of her own ideas. But in the big-huge-fantasy-series-genre, the Valdemar stuff's political worldview was pretty unique ten-fifteen-twenty-whatever years ago.

(I'm kind of phrasing this carefully because I don't actually feel things have gotten much better, but then I'm also not reading much recent fantasy.)

Also, she provided angsty slash pre-internet. I suspect that that gave her a boost.