It's extremely important to draw a distinction between cool and practical. HN is all about the cool new hotness. People here are always on the bleeding edge of technology, so they tend to flock towards those instead.
Also, hiring PHP developers is easier than <insert cool technology here> developers. There are just more people with PHP knowledge. Not saying that is a good reason to use a language, but it is a factor.
But when it comes to hiring _good_ developers, Python/Ruby/Lisp are definitely a better option.
In my experience, while it seemed at first that hiring PHP developers would be easier, it actually proved wrong, as we had to spend way more time to find _good_ developers among the mass than we would have if we were looking for, says, Python developers.
My personal opinion about this is that the better/more driven/valuable developers are the ones who are quick to try new tech and follow emerging trends. Many people know PHP, but I think you'd likely find better talent in the long run if you offer a position that uses a more exciting back-end.