One reason is that our thinking process is inherently serial and we do really badly at multitasking naturally. At least those involving deliberate thought. And

One reason is that our thinking process is inherently serial and we do really badly at multitasking naturally. At least those involving deliberate thought. And our programs are almost an extension of our way of thinking, so we will have to push the boundaries of how we think and model systems in our mind before we can build excellent parallel programming languages. Not that it isn't being done, but the weight of the "serial" legacy is long...