Because calling OS X the tenth revision of MacOS is silly marketing BS.
OS X is not OPENSTEP and it's not MacOS. It borrows some technologies from both and adds plenty of new ones like a new kernel, new display server, new driver architecture etc.
It ought to be called Oh Ess Ex Version One
Remember Apple tried to market SCSI as being pronounced "sexy" and we rejected that too. You don't have to swallow Apple's marketing foolishness.