They don't write one because of the size of the userbase. They end up supporting 99.99%+ of their potential customers by having Windows and MacOS support for something like a drawing tablet; the amount of money to support Linux doesn't justify the potentially low-four digit amount of customers that would buy the product if it had Linux support.
As a Linux power-user and the tech referral of my family, extended family and friends, if something works nicely on Linux I am obviously going to recommend it to everyone. That has to mean something...
It still either has to be a passion project for an engineer or it has to make business sense. Even if it only takes a collective week's worth of work in a year to keep it supported, that's 1/52 of an engineer's salary, and that has to be made up/exceeded by profit generated from the extra sales. Regarding this product, pens and drawing tables for personal computers are already a relatively niche category, so the subsection of those users who also use Linux is probably very small.
