There is a strong correlation between things whose primary design criterion include "any idiot can install", and "complete crap design".

As examples I hold up PHP, most PHP applications, MySQL, Matt Wright's script archives (a series of security holes masquerading as useful Perl scripts that were popular a decade ago), and so on.

The correlation is hardly absolute. For instance SQLite is both excellently well designed and also designed for easy install. However it holds often enough to strain coincidence.

My suspicion is that part of it is that something that is designed to be trivial to use tends to have a low barrier to entry to get involved with. This leads to getting contributers with more enthusiasm than skill.

Early on, PHP had the advantage of mod_php, which made deploying PHP applications simple and performed well. Because other languages required more complex setup (e.g. fastcgi, servlet containers) shared hosting providers gravitated towards supporting PHP applications. This caused open source projects that used PHP to gain more traction than projects written in, say, Perl. Eventually, hosting providers realized mod_php was not such a good idea. See the following link for an expansion on that http://www.majordojo.com/2007/11/is-mod-php-falling-out-of-f.... I think the shift of shared hosting providers to FastCGI (which makes supporting additional languages less work) as well as the decreased cost of private servers due to virtualization is playing a role in PHP's apparent decline in popularity.

Before there were lots of easy-to-use self-publishing apps, it made sense to install your own on a $5/month host. Most people nowadays use Wordpress, the service, not Wordpress the PHP package.