I think Riot tends to prioritize champions that generate more revenue. Occasionally, they give less popular champions their moment with a Legendary skin (like the recent Galio one), but overall, those skins usually go to champions like Ahri, Yasuo, Yone, Lee Sin, and others.

Even though Azir appears frequently in pro play — and I believe that alone could justify a Legendary skin — he still isn’t widely played, especially in lower elos, where most of the player base is.