Partially because if you use Xcode and Swift the CPU is mostly irrelevant. If you don't use those tools you'll need to care more about the CPU.

And partially because Apple built their own silicon, which means there could be secret instructions or instructions that work slightly differently than expected for ARM. I don't know how true this is for the M1 but it's been one of many issues holding back non-Apple compilers and Linux on Apple mobile devices. Having the M1 on a real desktop computer with deep debugging tools may mean we can fuzz and fully characterize the M1, which was harder to do on iPhones.