Why no Azir

Currently, he's not "meta" which is whoever is most powerful this season. This obviously goes to people like Fiora and Gangplank, and why many, many, many, many games will never be truly balanced (i.e This game, DOTA, most fighting games, some FPS and TPS games, all JRPG's can be broken).

Because nerfs.Riot destroyed him.Also they didn't give mana buffs 5.22 patch.Very weak for early game and normal for late game.Competely useless.

There's better options, that's all...

The AP item changes heavily nerfed him, over everyone else. This might just be me, but RoA isn't good on him, FQC isn't good on him, and Abyssal Scepter isn't good on him. Azir wants to rush Nashor's Tooth, but that's just not gonna work in this meta. Compare that to the picks we've been seeing: Ahri, Lissandra, Ryze, Lulu, and other mages that scale off of more average stats, ones that can use the current meta items. Now combine that with his repeated nerfs, and you're getting a below-average pick that can maybe zone a little.

In mid lane burst is always better than DPS, and Azir currently is strictly DPS. Before he got nerfed he had really good burst on his Q along with great DPS, now that his Q damage is nerfed, he's all about DPS, and if you're gonna pick a DPS champion, you might as well pick ADCs. If you want burst, then you'll pick Viktor, Ahri, Lissandra, or Leblanc, which is why they're in the meta.

You might ask "Well why is Lulu and Ryze in the mid meta then? They don't do too much burst". Well that's because Lulu has ridiculous supportive buffs while Ryze is just plain strong, although the low win rate might say something about Ryze's actual strength and the mid lane being burst preferred over DPS. Plus, the only person able to have strong performance on Ryze is Faker, so I wouldn't call him all that strong.