There are books on black tea, and books on matcha. Bancha has two scholarly volumes of its own. Put hojicha into a library search box and back come magazine articles and papers.
The reason is not known; the absence is
The reason is not known. What is known is the absence.
Searched across three routes
Searched across three routes — National Diet Library Search, CiNii Books and Rakuten Books — not one general book on hojicha as its subject came up. Six books carry "hojicha" in the title, and five are confectionery books setting it beside matcha or black tea. The one that does have it as its subject, Honma Setsuko's Hojicha no Okashi (ほうじ茶のお菓子, 2019), is a confectionery recipe book. One more borrows the title for literary essays (Tokiwa Shinpei, Atsui Hojicha (熱い焙じ茶), 1993). Neither the 608-page Encyclopedia of Tea (茶の事典) nor the 242-page official textbook of the Japanese Tea Certification has a chapter of its own (as far as the tables of contents go).
That it does not sell, and so does not get published
That it does not sell, and so does not get published, is a possible explanation. The figures for checking that are not to hand: the Research Institute for Publications keeps both the sales value and the new-title count in its paid annual. It is not there, so it gets written. That is all.
One thing worth watching while counting
One thing is worth watching while counting, though. "Hojicha" is also the pen name of someone who writes and draws. The number of volumes a search returns adds up once the author's name is read too.