Raw sales numbers are going to be biased because ARM is like Zerg and x86 is Protoss. In the ARM world there isn't the same concept of a central processor so normally lots of chips get built into each individual device.
ARM has also been historically used most often on proprietary systems you need authorization to develop for. So it's made less sense as a target for open source tooling hack projects like this one.
ARM also has so many sub-targets that it's almost like a coalition of ISAs rather than a unified one like x86. So adding ARM support to Actually Portable Executable might not be as simple as including an ARM build in the binary. We might need to have multiple ARM builds for its microarchitectures. Because ARM users want resource efficiency and they're not going to be happy with a generalized build that broadly targets ARM; they want code that's narrowly targeted to the specific revisions of the processor that they're using.
In other words, we can't give ARM users portable binaries because ARM users do not want them.
I also always thought that code for other architectures was the kind of thing that mostly got contributed by the people who build those architectures. Things like how IBM always graces our GitHub issues with patches each time our code doesn't work on s390x mainframes. I like that they do it by contributing patches rather than the feedback of why don't you support this? Why don't you support that? Oh I didn't say I actually needed it.