Java, Lisp, etc. have multiple implementations too. Different implementations often have different characteristics -- performance, compliance with the standard, etc.
In the specific case of C++, standard compliance was probably a pretty major factor, as the standard library for C++ is very complex, and vendors are not necessarily always fast to keep up with new features (or willing to port these to older platforms). For quite some time, STLport <http://www.stlport.org> was a life saver for providing modern, efficiently implemented library features on not-so-modern platforms.
Conversely, the fact that the Apache library hadn’t been updated since 2008 probably means is that it’s not C++11 compliant at all, significantly behind most vendor libraries, and the effort for upgrading would be fairly significant.
