I think part of the reason why many programmers haven't wanted to deal with parallel execution is because concurrency is not easy to handle. It has several pitfalls and can be painful to debug. Also, it needs proactive efforts implementing it, so as long as not required, devs just stick to serial execution.

Now, with helpful systems like no-side-effect functional languages and reactive stream frameworks, a lot of gory detail can be abstracted away. I think this has recently lead to more parallel-by-default software development.