The development was heavily influenced by what particle physics experiments required (developed at CERN and Fermilab so that makes sense), so the field naturall

The development was heavily influenced by what particle physics experiments required (developed at CERN and Fermilab so that makes sense), so the field naturally uses it essentially exclusively. It makes it easy to store C++ classes on disk, so after C++ was adopted as the language of the field post-Fortran, it became the natural tool. The event processing, fitting, and plotting tools were part of that “what particle physics experiments required” line too.