I believe reasoning goes something like this:

• a C implementation would likely be faster
• C is more portable (see. compilers are available on more platforms)
• C has more stable standards (or at least, more widely adopted)
• C compilers are more consistent
• lastly, Larry, Rasmus, Guido and Matz also chose C for their own reasons

Of course those points can be quite contentious, but I think they all hold enough truth to be valid reasons for not choosing not to use C++ for implementing the likes of perl, php, python and ruby.

I don't think it's a failing of C++, but it would seem that it doesn't really fit into the problem space of a language implementation. If you're interested to see what perl would've been like in C++ then Google about for chip's Topaz.