One reason might be the ability to hire quality developers in sufficient numbers. The BBC is big, with developers (both internal BBC and external contracts) working all over the UK on a wide range of output.
That's what I was thinking. Ruby, Perl and Python are all fine alternatives if you can find people with those skills.
... or train them.
and any good developer can pick up and be proficient in PHP quickly
which is in my opinion PHP's greatest strength and greatest weakness. Weakness because not-so-good developers think they are good while writing code using register globals, no input validation, no escaping output, etc..