Because experience from past mistakes.
Mill are doing some insane IPC improvements and also hammer out a lot of the security misfortunes that x86 has (such as the ability to perform a stack overflow into a return address or the ring security levels 0 and 3 that are so popular).
The EPIC Cpu that Microsoft worked on had Windows 10 and Linux running in the end. Including LLVM, C/C++ runtime, Busybox, FreeRTOS, .NET and core parts of Visual C++ 2017.
One of the core reasons that Itanium sucked ass was IMO that the architecture was too different and compilers as well as programming languages weren't taking advantage of it well enough to make the difference.
The network effect of "run existing blobs" is strong enough that I consider any change from ARM's RISC on the mobile space in the next 10 years unlikely. On the desktop the situation is similar but IIRC MS has been tinkering on a fairly good x86 JIT emulator for Windows which was able to run MS Office fairly decently, that might remove such network effects.