It isn't the default version because currently it can't be booted from.
ReFS is a pure storage-style File System, and it's been full of headaches since release for those trying to use it as such. Comparing it to APFS isn't really reasonable as they're meaning to accomplish two different goals both systems. (Keep in mind, some of the bugs that ReFS had included silent corruption on files over 2 TB, which is kind of important for even the upper-end of the small business spectrum when it comes to data. [1] Even with that many technical previews and idling in beta for many iterations, they missed a pretty big show-stopper for ReFS, which has no built in volume repair since it's not supposed to break. (To clarify, there is Integrity Streams, but it's more that if you still see corruption with Integrity Streams running, MS has no more tools in its kit to assist)
I get what you're saying, you don't want to watch a FileSystem do the 200 meter, you wanna see it's averages across several marathons, and I agree. However, what is important to take from such an install base is that so far the reports have been embarrassing implementations of stuff around the filesystem, not the filesystem itself not doing what it should. The fact that users aren't howling about data silently corrupting or extreme system slowdowns is a fairly good portend thus far, Apple's sloppiness on the surrounding code not-withstanding. APFS being stable doesn't excuse the rest of Apple's mistakes, but I would say separate the two; it's pretty clear APFS was a pretty important project for them, and it's clear where their attention was.
[1] - https://blogs.technet.microsoft.com/filecab/2017/01/30/windo...