A lot of those commits are related to features some people might not care about. For example, I imagine a lot of work has been done on the JavaScript compiler for QML - but if only use QWidgets, that's irrelevant to me.
Actually I'm only talking about the qtbase repo (core, gui, widgets, network...) in the Qt case, while for copperspice since they forked before 5.0 it's still a monorepo and the commits encompass everything (eg same that for Qt, plus script, WebKit and a few others they kept).
Copperspice forked at the time of Qt 4.8 and has had since then a couple thousands commits from a dozen contributors - in the meantime Qt itself had literally tens of thousands of commits, bugfixes, new features, etc from hundreds of contributors