Make sure you open all the treasure chests while levelling

You may just get lucky https://eu.battle.net/forums/en/wow/topic/17614613737

The devs have stated this since the beginning, that you'd get legendaries from just playing the game. You just have a higher chance when you do harder content. Thing is: leveling up with vs not looting the treasure chests is massive, while the chance for a legendary drop is almost non-existant and the amount of AP is very low. Whereas in a bit we all gonna have flying, and with higher AK levels the AP reward should be better vs time spent finding them.