There was Mac OS 9, though... though I think the reason why they called the next release Mac OS X instead of Mac OS 10 is because they actually made a legitimate big tech leap between 9 and 10, and they wanted to market that accordingly. The entire kernel's been rewritten from scratch.

Mac OS "Classic" really was an OS of its time, lacking memory protection and even preemptive multitasking - it looked cooperative as apps could run in the background, but all the preemptiveness was really a bodge on top of the GUI system, as the event loop INSIDE the application was required to hand over control back to the kernel to get new events; an infinite loop would hang the machine unless you manually handed back control to the OS using MacsBug. Many parts of the operating system were also stored in ROM, though it was designed to allow a newer Mac OS version to replace routines with its own. (This is why you can actually store a basic, but fully functional Mac OS system install on a single floppy!)

Mac OS X did away with all that 80s cruft, and is based on the unixoid NeXTSTEP kernel, with some BSD stuff mixed in. For backwards compatibility, it retained a lot of the old Mac OS "Toolbox" API (which was really direct kernel access for everything, including GUI) in the form of the Carbon API, though it was not binary-compatible. Given the unixoid kernel, applications run preemptively and under memory protection.