The PC platform did not get off its DOS compatibility crutch: while the 80286 had segmented protected address modes, they were incompatible with the "real address mode" that the CPU started in and since Intel envisioned real mode only to be interesting for booting, switching to protected mode was not reversible ("triple fault" as a quick way to get back to real mode was more an accidental discovery than a planned feature). Real mode bogged the system down to somewhat more than 1MB of memory (expanded by chipset via EMM memory into what the processor could have addressed perfectly fine in protected mode) and running multiple applications on top of a Window system was not much of a priority given the large constraints.
And the operating system basis, DOS, did not even do cooperative multitasking: pipes, obvious candidates for that, were implemented by writing the output of the first program to disk and then running the second program with input from the temporary file created by the first. This was actually worse than what MP/M (the successor of CP/M, the "inspiration" for DOS) could do.
While there were some multitasking systems built off the protected mode of the 80286 (like Xenix and Eumel/Elan), those were not mainstream.
On the Apple side, Motorola processors did not bother a lot with memory protection. While a separate MMU was supported from the start, it was optional and consequently rarely available except for machines explicitly intended for running UNIX. The 68010 was actually the first CPU that saved enough information at a page fault to restart instructions: previous versions were not suitable for running copy-on-write schemes and similar demand-paging features.
However, the 68012 with a larger physical address space and the 68020 with 32-bit data busses throughout and quite extended addressing modes still did not come with a built-in MMU. It's just with the 68030 that the MMU became an on-chip feature (and with the 68040, the mathematical coprocessor, at least to a good degree). However, either were considered mostly overkill for home computers.
Intel was faster with the 80386 which had a 32-bit mode (the 68000 was basically a "32 bit" mode from the start) and a fully virtualizable processor with a paged MMU. This was so completely overkill with regard to the active market Intel was pitching that I have no idea how the engineers managed to push this approach (rather than integrating a mathematical coprocessor, for example) through into silicon. Uptake was rather tepid at first, but it was what started Linux off the ground after a few consumer-level UNIXes like "Interactive Unix" and the proprietary clone "Coherent" were coming about.
Windows 95 eventually got to use the 80386 modes as well, but it required significant engineering and defining a Windows-3 like OS interface over the 32-bit protected mode. The consumer Windows versions were bogged down by history, and just with Windows XP finally the more capable Windows NT approach replaced the consumer line completely.
Apple had in the mean time successfully relied on cooperative multitasking, relocatable code, relocatable memory allocation and was getting long in the tooth with MacOS. In fact, they were in dire straits, finally breaking free by basing MacOSX on a BSD running atop the Mach microkernel. They managed to pull off this rather audacious move (under Jobs) while Microsoft spent decades getting its operating system struggling through compatibility issues. Later on, Mac also managed switching to different CPUs (first PowerPC, later Intel and ARM) while Windows never managed to crawl away from binary compatibility with DOS systems.
So in a nutshell: preemptive multitasking needed additional hardware for a long time, multitasking at all was not really in the DOS/Windows scheme for a long time, impressive built-in memory protection in consumer-level devices came late with the 80386 and even later with the 68030, and MacOS was rather successful with its cooperative multitasking schemes and almost missed jumping off.