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.
If you want to see a really good example of a multitasking GUI OS on 8086 check out Geoworks.
I also ran Desqview on my 286 20mhz which could even run stuff in re-sizable windows provided it was Desqview aware or would do BIOS screen writes.
I guess those ran on top of DOS, so were not really OS, or you could say DOS is their TripOS
Even OS/2 would multitask on a 286. Just not many 286s were sold with enough RAM to run OS/2.
