Chronos is just an example. There're many bugs in Mesos that don't get fixed for months/years. Mesos core is legacy (pre 11) C++ code nobody wants to maintain.
Disclaimer: Apache Mesos committer/PMC
This is actually very very VERY important. Go is a lot more concise (IMO) that C++, generally when I'm curious about how something works in a project written in Go, its much easier to follow the logic.