Aarch64 has taken off in the data center too. ARM is king of mobile, competing hard at the top, and thanks to Apple is a solid competition at everything in between.
Now, why does it not take off elsewhere? Microsoft's ARM implementation kinda still royally sucks ass. Also, the hardware still kinda royally sucks ass.
Are these being improved upon? At least the latter, yes. ARM themselves are pushing for what they're calling "SystemReady", where ARM based systems use UEFI+ACPI for initialization. A huge part of the problem leading up to this point is requiring board-specific initialization code being provided by the OS, usually in the form of U-Boot. So this meant there was just as many copies of an OS as there was boards. "Oh, you're on a Pi? That's Ubuntu #1. You want a SolidRun Honeycomb? Ya, that's Ubuntu #2 now"
SystemReady solves this by making it just as easy as PCs, where there is a "BIOS" so to speak that does that early board initialization and then does a hand-off to the OS to handle the rest, much like how X86/AMD64 works. This has been the hold up, and is what is being heavily worked on and improved now.