I think you mean, "The success rate of "QB's coming out is incredibly low."
Of all the positions in football, there's a good reason QB is the most difficult to project to the NFL (actually, there are several), and there's a good reason that number of starts is one of the important indicators to future success. By coming out early, of course, you're missing out on a whole season to refine your understanding of the game and pick up on some collegiate-level defensive nuances, which can then (hopefully) be translated to picking up on some NFL-level defensive nuances, and as if the game weren't hard enough, you're making it that much harder on yourself.
But money, injury, etc.
Even so, most senior QB prospects fail, as well. If juniors are more visible of doing so, it's because most junior QB prospects don't come out unless they're going to be a first or maybe second round pick, and those busts are always visible, senior or junior, whereas you'll get seniors taken later who do better. But I don't think the rate of successional seniors is too much higher than that of successful juniors.