I've never understood why the height of a QB matters???

I thought his job was to step on the field, throw TDs, & win games....not try and improve your height. I just read an article about a QB @ the University of San Diego named Josh Johnson who said he wasn't highly recruited out of high school because of his height! Since when does it matter how tall a QB is? I don't hear anyone complaining about Fran Tarkenton or Doug Flutie, so why is this an issue?????