It doesn't support a lot of modern Wi-Fi chipsets. There was an entire wave of Broadcom-powered stuff that they weren't able to develop for.
It's a network problem. Manufacturers aren't gonna care unless it's a big OS with lots of users. Your OS won't get a lot of users unless it has drivers. So the result is stagnation, and only the Big 3 OS continuing. Well... really the Big 2. Mac OS is a unique situation.
[0] https://lore.kernel.org/all/20070405163434.GA18477@gollum.na...