Let's face the facts - there are just too many PHP CMSes out there for people to choose from. Half of them are too big and bloated to really be of any use (you know who you are) and the other half seem to be more half-finished projects than anything else. Yes, there are some shining stars out there, but they seem few and far between. Thankfully, the overall quality of the PHP CMSes seems to be higher (in popular opinion) than a Java Content Management System - and this thread from indicthreads.com gives some of those reasons.

One question that keeps being asked about IndicThreads.com is that how come you are running on PHP when you are supposed to be a Java J2EE portal?

Honestly, we would have loved to be on Java and be able to experiment on the site using Java software and also keep generating in house articles about how we manage IndicThreads using X component and how we upgraded to Y server and so on.

Some of the reasons that they give include the fact that PHP CMSes are "more evolved", they are easier to use, hosting them is usually simpler, and there is usually a large community surrounding each of the PHP CMSes.