Linux, and much of the open-source software that goes with it, has been multi-architecture for a long time. If you take something that already runs pretty decently on x86, x86_64, PA-RISC, Motorola 68000, PowerPC, MIPS, SPARC, and Intel Itanium CPUs, porting it to yet another architecture is, while not trivial, at least mostly a known problem.

Windows, by contrast, was built for descendants of the Intel 8088, period. It’s unsurprising that porting it is a hard problem and that results aren’t always satisfactory.

(Apple built on top of a modified BSD kernel, and BSD has also been ported around quite a bit, so they also have a ports-are-a-known-problem advantage.)