It's not entirely true. Copland microkernel was in (hidden) in System 8.5 onward, and provided preemptive multitasking, file mapping, timers etc. It's just that the only real 'task' on it was the old cooperative OS.

They had made a LOT of work on that, and the driver model by the time System 9 arrived. What really failed on Copland was the 'userland' equivalent. At the time, Apple had gone into a completely bizarre way of designing complex APIs for everything and most of them had no use whatsoever. There were heaps of crap, like OpenDoc and many others and Copland was 'trying' (and failing miserably) to integrate all of that.

How do I know? I have a collector t-shirt with 'Copland Driver Kitchen' on it ;-)