The easiest way to put it is probably to say that C is just not the right language for a system in which everything happens at once. You use a programming language to tell a CPU what to do, and you use an HDL to tell an FPGA what to be. An ensemble of explicit state machines is a better way to picture it.

If anything, a pure functional or declarative model might be a better starting point for high-level synthesis than a C-like imperative language. But most of the R&D seems to be going in the opposite direction.