People go to the shiny sites. If they see older-looking sites, they're less likely to stick around, particularly if it doesn't have the nice features that the newer sites have.
For all the problems that PHP has, I don't see many nearly as many sites going up built on other platforms, in large part because they're playing catch-up and are still largely years behind. .NET is probably the closest, but when you look at the number of free or even inexpensive sites running Windows, it pales in comparison to the PHP-based sites.
Add to this that WordPress is by far the easiest of the major CMS platforms to manage, and it gets even worse. I manage a couple of WordPress sites and a Joomla site. WordPress largely Just Works(TM). Joomla works for basics, but every time I want to get beyond adding a menu item, it becomes a whole new learning process.