One of the reasons is that the first Intel processor to support multi-tasking was 386. When DOS was created, there was no hardware support for multiprocessing and virtual memory.
For designs that wanted an MMU, you could use an external one. Sun had external MMUs in their early workstations. Even Apple had their own MMU in the Macintosh II line before they switched over to the 68030.
You could add a 68851 to a Macintosh II, I didn't think Apple developed their own MMU for it. The Lisa had a custom MMU.
"The machine shipped with a socket for an MMU, but the 'Apple HMMU Chip' (VLSI VI475 chip) was installed that did not implement virtual memory (instead, it translated 24-bit addresses to 32-bit addresses for the Mac OS, which would not be 32-bit clean until System 7)."
Not quite. The 80286 was not a good CPU, but it could do multitasking:
https://www.cpu-world.com/CPUs/80286/index.html
> The second generation of x86 16-bit processors, Intel 80286, was released in 1982. The major new feature of the 80286 microprocessor was protected mode. When switched to this mode, the CPU could address up to 16 MB of operating memory (previous generation of 8086/8088 microprocessors was limited to 1 MB). In the protected mode it was possible to protect memory and other system resources from user programs - this feature was necessary for real program multitasking. There were many operating systems that utilized the 80286 protected mode: OS/2 1.x, Venix, SCO Xenix 286, and others. While this mode was useful for multitasking operating systems, it was of limited use for systems that required execution of existing x86 programs. The protected mode couldn't run multiple virtual 8086 programs, and had other limitations as well:
> 80286 was a 16-bit microprocessor. Although in protected mode the CPU could address up to 16 MB of memory, this was implemented using memory segments. Maximum size of memory segment was still 64 KB.
> There was no fast and reliable way to switch back to real mode from protected mode.
More about OS/2 on the 80286:
https://www.landley.net/history/mirror/os2/history/os210/ind...