Riot wants to remind everyone that they should be playing their precious ADC's. Have you noticed 3 of the 5 champs are ADC's?
Why have a mage (or true AP champ) featured in their action-packed e-sport? Their cooldown-based gameplay is too slow and boring. Yuck!
Look at the artwork for Nexus Blitz and tell me which champs you see.
What's that, you say?
The only two supports Riot cares for (Blitzcrank and Thresh), multiple ADC's (again), an AD juggernaut (Darius), a tank (specifically Galio because of the current Galio phase they're going through), and the only real AP champ (Teemo) being blown up in an undignified manner ("this is what happens when you play an AP champ in our game."). Of course Thresh isn't affected by the explosion because he needs to look cool enough to pick up and play.
Nexus Blitz is all about fast-paced, non-stop action, right? So featuring a mage would be counter-productive to their agenda.
Remember this login screen?
They could have included any champs in that, but who did they go with? Why, their golden children, of course!
The evidence has been around for years. Welcome to League of AD.