Board Thread:General Discussion/@comment-26492438-20150613104655/@comment-6881629-20150616125020

Tiber Septim had the Dragon called Nafaalilargus as part of his army. As for other things a non Dragonborn wouldn't able to use the Elder Scroll to learn the shout to weaken Alduin, they probably wouldn't have been able to convince Odahviing they were worthy of his help either (espesially without Dragonrend to weaken and trap him).

Also the often used reason of prophecy.