Ekkos buffs.

The buffs are meant to help Ekko they way he was intended to be played: as an assassin. The cooldown on his E when play assassin Ekko constantly inhibits his killing ability, along with the early costs on his Q and W. The reason people play as Tank Ekko is because his burst damage is too slow in comparison to other assassins like Zed or LB. That way he offers far more to his team as a Tank. The buffs weren't meant to help him as a tank, and his tank form isn't that oppressive unless his team can capitalize or the enemy is getting caught by his ULT. Going tank also makes his Ult more dangerous because it scales with how much damage he's taken in the past 4 seconds.

I'd also like to add that if you look deep into Ekko's mana costs early, running tank is even more of a risk early without any damage. Unless he gets fed, all he can do is skirmish, cause he sure as fuck cant kill anything.