Reference: ttps://en.wikipedia.org/wiki/Usage_share_of_operating_systems#Desktop_and_laptop_computers
My point is, that 10% figure is misleading. China is higher, everyone else is lower.
It's probably mainly because Vista has a relatively low market share: probably lower than XP. It's thus not worth supporting as the earliest version of Windows they support.
Edit: Just checked. Windows XP lost non-extended support in 2009, whereas Vista lost it in 2012.
Of course, that gets complicated when you consider something like Chrome's sandbox, which can depend on system guarantees that Microsoft may consider esoteric or just low-priority. To their credit, Microsoft is responsive when we find these kinds of issues and report them. However, it's not uncommon for the issues to not be considered important enough to backport to legacy OSes (or for the task to just be too arduous). That's why we also tend to focus our work on the most current supported versions, which also offer increasingly better mitigation technologies with each release.