Just a guess here, but true object-orientation is really hard to get right. It's not like OS X was going to be built on Smalltalk. And if you're going to half-ass OO with an implementation like structs+functions, you're usually better off sticking with a functional system. This is especially true when it comes to high availability services, like operating systems.
Given the above link, I deny there's any single useful definition for OO: It's been defined and redefined over and over again for decades, to the point people can't even agree on what languages are capable of writing software in an OO style.
