Apple is the backing force of ObjectiveC.
However WebKit is written in C++.
Apart from portability (not all systems have ObjectiveC compilers/runtimes) is there any other valid reason for this? Performance, features?
Lately Apple does not seem to care of other languages than ObjectiveC.
Aside from portability, the use of C++ is an historical artefact. WebKit wasn’t started by Apple; it was a fork of KHTML, which was developed as part of the KDE project. The decision to use C++ was made by the KHTML team, likely because KDE itself is written in C++ using the Qt framework.