My immediate guess as to why there are not many 6'+ girls in porn would be because the guy's package would look really small (i.e. it's true size). Just browsing through stats I've noticed the average height women in porn is between 5' and 5'6", which of course means the guy's hard-on is going to look monstrous with those girls. It's all about the preception on camera.
Which is why whenever I see those adds that say things like, "GET A BIGGER DICK WITH THIS ONE WEIRD TRICK!", my first reply is, "Date a really small woman?".
