The ARM server market is mostly handwaving bullshit. Why do you think the big cloud guys are gobbling up Xeon D? It hits the low power market without being a ridiculous construct with 32 cores that needs eight separate memory controllers and it integrates proven platform features such as Intel 10GbE.
Okay, this is where I think it gets complex. I agree with
@Ericloewe about why Xeon D is popular. There's a lot of desire for familiarity within the IT/cloud world.
We're at (and have been at) a stage in "cloud" development where a lot of what's running is no longer Microsoft Windows. People are writing in scripted languages, Python, Java, Node.JS, PHP/XHP, whatever. If you can compile the interpreter/VM, guess what, no one gives a crap what silicon's running underneath. Often that's also true for a compiled language (C, C++, whatever). Hypervisors are a vaguely has-been technology as we move on to containers. Big monolithic CPU has been giving way over the last 20 years to better design by programmers that allow things to be broken up into many bits and pieces to be spread across multiple cores or even an entire network of machines. These things are all things that can work in favor of ARM.
An ARM processor
with 64 cores or more could conceivably be a really big deal. Intel's stuff is "good", but the ARM stuff is more efficient overall. It COULD be a killer if only it could get a foothold. But in order to get a foothold, you'd need some serious adoption, and then you'd need to see it actually available on commodity server boards, and we're just not anywhere near seeing this, as far as I can tell.
Either Intel has to fall behind or ARM would need to make a massive improvement of a sort that just doesn't seem to be forthcoming at this time. Until then, it seems like no one's really interested in being an early adopter of a technology that just doesn't really offer much benefit in its current iteration,