The difference is that major Mac software packages still use Carbon, e.g. Office and Photoshop. You can't drop support for Carbon because of those.
I'm sure Apple would love to simplify their lives by removing Carbon, but any version of Mac OS X that doesn't run Office is not a version I can ever use.
Since I believe they've dropped the Lynx trademark having this as the last cat and dropping PPC/32 bit/Carbon next time would make some sense, and they'd need to give a lot of warning for doing that.