Because they forked it from WebKit which is derived from KHTML, which has a history that stretches back to 1998. Better to ask which languages would have been a better fit for a web browser in the context of the late nineties that have had the staying power of C++ (I'm guessing you'll come up with a rather short list).

If you want to see an attempt to write a new browser engine in a language that is memory-safe by default, I suggest you check out Mozilla's Servo engine: https://github.com/servo/servo