Earlier quoted context omitted.
The Parallax P8X32A Propeller (2006) did multi-core processing in a very beginner friendly way. It can be done - if you take a holistic approach to hardware + runtime + development environment. The Propeller probably failed because of the custom language, the custom assembly syntax, the custom ISA, the custom IDE font (!) etc. It was a very neat system though.
The Propeller was a commercial failure because it was a one-off part, from a small company, with very little software ecosystem surrounding it, a poor performance to price ratio, and no migration path if you needed more capabilities than it could provide.
It was just too unusual in too many ways.
In one way it’s a bit like the Amiga vs the 8088/8086 PC.