Software.
The reason that you need a 386 to multitask is because everyone wanted to run their old dos software, which assumed it was the only program running.
It would be trivial to write a multitasking operating system for an 8086, however you'd have no software to run on it. There was no reason to ditch all the software and start again, because the 8086 wasn't quick enough to warrant it.
I don't think anyone realised just how big and important the PC would become, or it would have been designed better.
It's worth noting that the Data General NOVA and later the seminal Xerox Alto both ran on bit-slice ALUs, not even what we would think of as proper "CPUs".
Minix runs passably well on an 8086, although one needs to keep in mind memory issues (max 1mb addressable). While I don't
think you could run a GUI on top of that, you never know. There's a guy building a Mac-like OS for 8-bit Atari systems and it looks
slick. If that'd hit back in the 80s I'd probably still have one
