In spite of Microsoft's cryptic announcement of Project Volterra, and Qualcomm's continuous lineup of 'flagship' ARM SoCs for Windows, Microsoft is still behind the 8-ball when it comes to ARM.
Apparently, in 2016, Microsoft entered into an exclusivity deal with Qualcomm. That's why all official 'Windows on ARM' devices use Qualcomm SoCs. At the time, Apple hadn't yet pulled off its third major architecture shift for macOS, from Intel X86 to ARM.
Looking back, products like the Surface Pro X and the myriad ARM for Windows laptops, were basically built to a budget and for portability above all else. They were never competitive with Intel/AMD-based computers. Microsoft seemed to think ARM would always remain in a niche, only used for light, mobility-first devices.