Speed of running the code was likely why C++ was retained, Erlang is a higher level language that uses more resources, and depending on your platform target the

Speed of running the code was likely why C++ was retained, Erlang is a higher level language that uses more resources, and depending on your platform target there may not be enough CPU resources in your average base station to support a software stack in Erlang doing packet forwarding, QoS or especially DPI. That being said, for communications platforms it has some very useful properties when code starts to break, hence WhatsApp and 2600hz using it for their respective projects.