Because it was built with the newer design language that Apple was developing - they used it in Quicktime too. I'd hazard a guess that the key difference was that at the time that MacOS 9 came out, you could get something in as long as it appealed to Steve.

It's not that Apple didn't experiment in earlier days, but this was a time when the push to the next OS and design was really strong after so many failed efforts to get a next-gen OS out the door.