Those high level protocol controllers came later to speed the process up and allow debug without exposing ridiculous amounts of IP to the debugger vendor. Originally it was just access to the full internal scan chain.
It’s a little more to it than that... I think you are severely understating the necessity of “speeding up the process.” IP concerns aside, an internal scan chain may still not give you features such as practical monitor mode debugging with any useful level performance. The point is, debugging in general is better served by dedicated hardware, at that point putting it on the jtag bus vs any other serial bus is more a basic pragmatic issue.