It's a licensing nightmare, from what I've pieced together from old forum posts. The original novels were written by different authors (like Kouhei Kadono for 'PHF') but are based on Araki's characters and settings. That means multiple rights holders are involved: Shueisha, the author of the specific novel, and possibly Araki's studio. Negotiating all that for a niche (in the grand scheme of things) print run in the West is probably more hassle than most publishers think it's worth. The anime's popularity might change the calculus eventually, but for now, it's stuck. It's not like a standalone light novel series where one publisher holds all the cards; this is a tangled web of IP agreements.