Seth Gottlieb wrote a great post looking at The Onion moving from one open source CMS to another.
One driver is that The Onion has developers on staff, which strongly influenced the systems that were attractive to them above and beyond the content management features. Seth points out their prior platform encouraged not tinkering with code under the hood while the new one is very code-centric. They also have an insane amount of traffic, which created a unique situation as well.
Good reading if you are into content management.