although the book (and its sequels) seem(s) to be well established
among the ranks major French fantasy novels
While the novel was originally published at the beginning of the 1980s, it gained some fame only in 2010, due to a new edition. Before that, it wasn't well-known, in part due to various publishing incidents which certainly didn't help relatively to a possible translation in English. From the wikipedia article about Jacques Abeille:
Les Jardins statuaires was to be published by Régine Desforges, with
whom the publishing contract had been signed. But the bankruptcy of
the publishing house L'Or du temps prevented it. Abeille then
entrusts the typescript of the novel to Julien Gracq, who then gives
it to José Corti, historical editor of surrealist publications. But
the typescript seems to have been lost and never reaches Corti's
hands. It is finally Bernard Noël who, several years later,
discovers another typed copy of the novel and undertakes to publish it
at Flammarion, where he is then editor. But a delay in production of
the book hinders its release, which also suffers from the departure of
Bernard Noël from his position in 1983. Various other events delay
publication of the novel, which inaugurates the Cycle des Contrées,
acquiring the status of a cursed novel.
No source seems to mention a translation in English. For instance, a 1983 review in English in World Literature Today only mentions the edition in French, and does not mention a translation in English. The Internet Speculative Fiction Database does not mention a translation in English either. Google Books search engine does not seem to know anything about Jacques Abeille's books being translated in English. So it's certainly safe to assume that no English translation of the book has been published so far.
You ask:
If there are really no translations, was it on purpose (e.g. the
author's refusal, etc.)?
The closest related statement I found is in this interview from 2011, where Jacques Abeille explains that he wishes his work to be kept somehow "confidential" (26:26 to 27:30, automated translation from French):
I'm in an almost impossible situation, because I do write, but I want
what I write to have an almost confidential value. Because that's what
I like when I read. I like the feeling that the book, and through it a
certain man or woman, speaks to me, of all people, and not to
the crowd. So that puts me in a difficult position insofar as the
conditions of the book industry today allow less and less for this
confidential tone.
He does not mention the issue of translation, but this quote might suggest that he could have been a bit reluctant to have his book translated in English. However, as mentioned in previous comments, there are translations of the book in two other languages (Spanish and Serbian), which in my opinion contradict possible speculations about Jacques Abeille's stance toward translation of his books in other languages, including English.
If you look at other possible reasons, someone else previously mentioned that it is rare for French fantasy novels to be translated in English. It is mainly due to profitability concerns from publishers. Even if there are exceptions, French fantasy is generally not selling well in France even nowadays, so prospective publishers abroad may be a bit wary of it.
Besides, publishers may consider that cultural differences between French native speakers and English native speakers could increase the financial risk. It could explain the absence of an English translation so far.
From this 2019 article by Lloyd Chéry, on why French fantasy does not sell well (automated translation):
A related problem with the omnipotence of the English-speaking market
is that, since it's self-sufficient, it doesn't look elsewhere. Even
the best French fantasy titles find it hard to get a translation to
make their mark internationally. "If you make novels that are too
original, the Anglo-Saxons are afraid they won't sell. And if they
don't, they think they've already got everything," sums up Imaginales
director Stéphanie Nicot. Interviewed for our special issue on science
fiction, Orson Scott Card also pointed out a fundamental difference
that doesn't help matters: "European literature is much more
descriptive and philosophical, it's a completely different way of
writing. Americans emphasize the story above all else: here's what's
happening and why. Whereas the French will describe, dress, use
metaphors and so on. Which makes them hard to read for an American
audience used to going straight to the story."
