In my opinion, largely because they own a bunch of industry standard formats, so any competitor would be forced to start by cloning one of their products. Once you've successfully done that, congratulations, you've conquered 4% of the market, in that one area (let's say a photo editor), and your software still seems broken (although it's faster, bugs get fixed quicker, and the UI wasn't designed by a crazy person or filled with 30 year old cruft) because it doesn't seamlessly integrate with the rest of their suite.
Affinity seems to have made some penetration by copying/innovating on an entire range of their products, but that's a huge up-front investment to grab a tiny piece of the pie.
This is really a job for antitrust, but modern antitrust is just a circa mid/early 20c legend we tell children so they aren't scared that capitalism could get out of control and eat them.
