I am seeing that Dartium version is 46.xxx which is far behind Chromium which is currently 56.xxx. As what I see, v.46 is about 2 years old now.
Is it possible to combine Dart VM with a more recent version of Chromium or somehow get Dartium which has are more recent version?
What is the reason for the Dart developers to keep Dartium version so old?
TL;DR You probably can't (easily).
From the phrasing of the instructions to build Dartium from source:
These are instructions to check out and build a modified version of Chromium that embeds the DartVM.
and further down
A Chromium / Dartium checkout pulls code from ~90 repositories into a single directory hierarchy.
I haven't delved into the source in a while, but it would require a team to maintain the changes every time Chromium is bumped, which is often and probably beyond the scope of what they want to do (since they didn't even bother doing a security audit of Datrium, also from link).
Furthermore, Dartium is not intended to be consumer-facing, but is strictly for developers, since Dart web apps are transpiled to JavaScript.
So, to update Chromium in Dartium, you'd need to port those changes yourself.