Linux

Because it's open source and most of the applications for it are open source. That means you can compile it and the applications specifically for the hardware you have.

Windows does kind of support ARM on its specific hardware, but it can't be adjusted for other hardware and they have to translate most applications to work. Apple has done much of that work for their hardware to work well, as well as very good translation for x86, and because they leaned hard into the transition, developers were mostly forced to compile for ARM going forward. Microsoft hasn't done the same, and ARM is a tiny target, so it doesn't happen with any regularity there.

Because people have been doing so for a long time and have ironed out most of the quirks. The software is also generally quite simple, meaning there are just fewer quirks that need to be ironed out. And the ecosystem is largely open source, meaning everything can be recompiled to target the relevant architecture, so while translation layers are still useful, they're not the essential tool they are in proprietary ecosystems. The main headaches that plague windows on arm mostly just don't exist on the Linux side.

Because it's not developed by some corporate fuckers whose only goal it is to make as much money as possible, it's developed by individual skilled people in their free time, because they're passionate. They don't want to sell some garbage, they genuinely want to make a good operating system for themselves and everyone else to freely use without any restrictions. FOSS is not about the money, it's about actually creating something good.

Because you can try compile it on arm, and if something doesn't work you can report it or fix it yourself. That said windows worked fine on arm years ago. Many gps, medical, and such devices used to use windows ce on arm, mips. (Windows phone too, arm)