1. Others with creative suites don't do it. You get a niche, and marketing value.
2. The codebase is, by and large, the same you'd have to run on Mac. This is how a number of video games are ported from Windows to MacOS (Tomb Raider, etc.).
What they "seem" to have in common with Affinity products is that they don't rely on a native UI toolkit.
3. Support cost can be reduced. Linux users support themselves: give them broad strokes, broad strokes would be enough.
4. Finally, the Linux market, as is obvious with 21 pages on this thread, is largely underestimated. Top design products, such as Houdini or Da Vinci Resolve, are distributed on Linux, because top professionals and studios like to work on Linux.
So-called "power users" or creatives like me dual boot, but wish we could loose the last anchors that tie us to Windows - or to Gimp - and embrace a reliable, a sound product for graphics editing that runs on Linux. I'll assume we're in the couple of tens of millions whose professional activity is not properly reflected in online usage statistics.