It's the ink manufacturer Toyo, the head office, in a phone conversation with senior staff there that's providing this limited answer as to why they stopped. At the suggestion that it was market demand (a decline in printed books and concentrating more on printed packaging), they said it was a failure to achieve consistent results across "a variety of substrates" and that printers were dissatisfied, so they abandoned it. I submitted that for very carefully designed and executed books, and with the knowledge we've gained in the centuries since Gutenberg's invention, we've learned a great deal about what makes for comfortable extended reading, and glossy text with glare is an impediment, particularly in lengthy scholarly works. Their explanation is also somewhat at odds with a response received from another regional branch, where they stated that although poor demand was behind no longer offering it as a standard option, if we ordered 800 kg, they'd still mix it, a min. quantity. In all exchanges, I covered with them the paper stock used and the equipment. Suffice to say there's major resistance. That's partly why an LLM explanation seemed enlightening: matting agents scatter light and interfere with UV curing. The stock was chosen for its inherent matte properties, which is now at cross ends with UV ink's gloss. I cannot do a matte varnish to the entire page as we wish to keep the virgin white paper free from a coating and the many images work perfectly with the process UV inks as is. We'd do a spot for text.
