Because PHP is so readily available (installed on pretty much every free web host), it may also be related to people starting out with programming and getting frustrated about it.
I use PHP daily and I like it's features and tooling but it really isn't a very good language (it's finally passable but far from good).
In addition to the reasons you mentioned it's also possible their are other demographic differences among languages in terms of age, experience etc. (I suspect though can't prove that the average C programmer is older than say the average PHP programmer).
