Take it up with your particular Linux flavor's repository maintainers. We don't actually maintain the Linux Sigil packages, but we do work with those maintainers when they have issues or reasonable requests. There are plenty of distros out there the offer quite recent versions of Sigil in their repositories. Unfortunately, some distros package management systems are a quagmire of slow-moving, overly bureaucratic processes that use "stability" as an excuse for maintaining mountains of ancient software. There's also a good deal of idealistic hair-pulling and teeth-gnashing over WebEngine that recent versions of Sigil require.
The bottom line is that any distro that has Qt5.9.5+ (including QtWebEngine), Python3.4+, and cmake 3.0+ in their repos could provide the latest version of Sigil if they wanted.
https://repology.org/project/sigil/versions
It has been my experience that most Sigil package maintainers are doing the best they can within the particular bureaucracy they have to work with.