And people just complain that life isn't easy enough. Psh.
The reason? Almost zero deployment complexity. You can take Joe Web Designer off the street, give him SFTP (or, more likely, FTP) credentials, and have him modifying your site in minutes.
Modify a file, click refresh.
I love PHP, but I'm in programming like a shade-tree mechanic: I just want a large enough hammer to make something work, I don't care how pretty the end result is.