As I understand it, “hanging, drawing, and quartering” was specifically designed in order to be torturous and frightening. Certainly drawing (whether referring to dragging by a horse, or disembowelment) and quartering fulfilled this purpose. I wonder, however, why hanging was a constant in this array of procedures.
I would think that the risk of premature death (as was the case during the execution of Guido Fawkes), and the potential loss of consciousness and perception which would diminish the suffering experienced in the remaining part of the procedure, would make this inappropriate for the intended purpose. I would also assume that strangulation in and of itself, while certainly excessively distressing by present standards, did not compare in either experienced or displayed suffering with drawing and quartering.
My best guess thus far is that there was a “ritual” aspect associated with hanging, so that authories were motivated to keep it as a constant feature throughout execution methods (here, however, I would remark that I have not heard of hanging being used in conjunction with burning), or maybe that it allowed broader viewing during public executions (this would explain why it was not used in conjunction with burning, since that would have been broadly visible on its own). Still, I was unable to find any sort of authoritative or historical commentary on this.