1. Some of mercurial's bundled plugins are not Py3 compatible (I recall reading a discussion as an outsider a while back that some plugins are planned to be ported to Rust, not Python 3, but that may have changed)
2. Nobody involved in packaging it for Arch has had time to verify mercurial 5 on python 3.
So looking into this a bit more (and my edit window has expired), it's because the mercurial team themselves suggest not to package mercurial 5.0 for python 3 yet:
