Chrome used GTK2 as that was current at the time, and when we looked into GTK3 we concluded that it'd be a pain to target both simultaneously (the point of the new major version number is that the API changed) and that as long as we were supporting old distros (e.g. "Long Term Support" versions) we should keep GTK2.
These days, those older versions are dying away, but Chrome doesn't use much GTK anyway. I wonder how hard it would be to target both. But that doesn't solve the binary compat problem anyway.