Why is Ekko so strong this patch?

He's always been strong. Just because a champion isn't played every single game doesn't mean they're weak.

People finally realized he is strong when built AP. And they got used to playing him

Hes not suddenly good, his play rate is just up more. http://champion.gg/champion/Ekko/Middle

Ekko loses to along with having bad times against all of these champs have either been directly or indirectly nerfed meaning the buffs given to ap ekko during the assasin update can start to show more since he has a safer lane to get to his ap powerspikes, also the overall damage nerf helps him out alot since he does have relatively strong defensive tools that require him to survive for a short period of time before activating, since it takes longer to burst he has more time to react, add to this the fall of the CoC tanks from toplane, who would stun him everytime he tried to make a flank in a team fight, and could duel him if he tried to split push means that the map is far more open to him