Chromium Embedded Framework is Chromium's XULRunner. It was much easier to use and maintain than XULRunner, which is why it's still maintained to this day, and XULRunner is in the trash.
CEF is a third-party project that isn't officially supported by Google, unlike XULRunner. Which gets back to my point about market share: when you're popular enough, then people will step in to provide embeddability (in this case, CEF) on top of your non-embeddable product (Chromium). You could easily imagine an alternate universe where Firefox was the #1 browser and someone stepped in to write a Gecko Embedded Framework. The reason why Chromium Embedded Framework exists and Gecko Embedded Framework doesn't is market share.