Because Chromium is much slower on old hardware and long to build.
I am not sure it is really a good idea, as written in upstream wiki:
alternatively QtWebKit - only the updated fork (5.212) is supported. Note: The latest QtWebKit release is based on old WebKit revision with known unpatched vulnerabilities. Please use it carefully and avoid visiting untrusted websites and using it for transmission of sensitive data.
So it support only a fork of qtwebkit, and this as security issues.
I didn't search that much but if upstream support an official version with less obvious security issue, we could add it to the ebuild for sure.
Please note that Qt5WebKit is deprecated.