Time line of Ekko.
around patch 6.7 tank Ekko top was so popular with like above 55% win rate, and mid was doing ok with 50% winrate. we also saw some good play in NA LCS spring final with AP Ekko mid.
But yeah tank Ekko top was problem(and i think he still is). than Riot decide to start Nerf tank Ekko and and buff AP ratios think it would solves the problem. But it never did. Right now Ekko top has 48% win rate and Mid Ekko with 45% win rates. Tank Ekko is still better.
Ekko kit very strong in general and it become more and more power full as he able to survive, which make tank Ekko always better than AP one. But Riot designers don't want Tank Ekko for some reason( i don't know why). It will be better if Riot accept tank Ekko and balance around his mobilty and mana resorse