Until a book is converted from print to a computer program for printing, it is entirely too expensive to redo the manual typesetting required to reprint limited copies of a book that would have such as specialist appeal. That is the main issue: cost vs recouping that cost and then, hopefully, making a small profit from the limited appeal base for the book. It is different with the internet publishing base; the books are collated and formulated on computers, then scanned into publishing software with all the decorative type font changes and position of the pictures. A test book or ten are run for verification of the program and then sent to the computer run high speed presses. Publishing a book with this type of limited appeal means a 'short run' use of the high speed presses. Since setting up the computer program and loading the program and verifying the perfection of the book is the expensive part of computer publishing, short runs are not good for maximizing profitability. And if you publish extra copies in the hopes of selling them in the future, you have to pay for the storage and tracking of the stockpiled books. Let us face it, money talks. Add all these factors together and it is probably cheaper in the long run to find and buy one of the existing copies.
Then you have the phenomenon of each existing copy of the book becoming more and more valuable as the numbers of the book available for sale dwindle...
Frustrating.