Why did Tanith Lee have difficulty being published in the '90s?

Lee's problems seems to start in the late 2000s. In the interview (from 2011, published 2012) that is the source of her claim to be unpopular among publishers, Lee claims that "I have at this time no new book, adult or Y.A, either out or due to come out, let alone any contract to produce a book for any of the main companies." Indeed, this seem to have been a low point in her career, with only one new novel and a collection of new stories published after 2007, not counting 7 coloring books.

However, if we look at her bibliography, there is no lack of published works from 1990 to 2007 - I count 39 novels, which is quite impressive (even if some of the novels are short YA books). I can't tell how many of the short stories in the many collections published after 2007 are new, but at least the Marcheval stories appear to be new.

Her problem with major publishing houses could feasibly derive from having had one or two poor-selling novels. Other once-major writers have found it difficult to find publishers once they have been branded poor sellers. It happened to e.g. John Brunner. It could also be that her writing didn't seem as original when she was in her 60s as when she was younger.

I have read all her interviews, books, short stories, and most if not all that has been said about her online and in media for decades now. The correct answer is that no one seems to know for certain, so we can only speculate.

We do not have public sales data; publishers hide all of that nowadays, so they can give bestselling awards to whomever they choose, usually talentless rich kids and political operatives. The film, television, gaming, and even our news industries run similarly today. Her writing was always original right up to the end, and many less creative writers stole ideas and even entire characters from her.

The publishing problems began well before 2007, and it seemed largely inexplicable to her as well as her many fans around the world. Most of her later publications were from independent presses. With the recent passing of Storm Constantine, who published Tanith through her Immanion Press company, there are fewer and fewer reliable witnesses to tell what happened.

Her husband, John Kaiine, might know, and I keep waiting for an in-depth interview for his opinion on many questions such as yours. I hope the people involved (not that many, really, the world of these decision-makers is claustrophobically small from what I have seen of it) come forward to tell a more complete tale.