Not all systems support qtwebengine (by support I mean: have the current version of qt), and the developer still has qtwebkit as a fallback. Both implementations differ a lot, and I'm happily impressed to see support for both at the same time!
However, there's usually an easy way to get an up to date Qt with QtWebEngine if you're on Python 3.5/3.6: https://github.com/qutebrowser/qutebrowser/blob/master/doc/i...