What's going on right now, from what I can tell, is just a lack of reliability from the way Assassins and ADCs are supposed to be built. Assassin Ekko is fundamentally one of the hardest Assassins to play in this game, couple with slow burst. His utility makes up for this, but people have eventually realized that it's easier to contend and contribute to the team by being able to survive instead of doing huge damage.
By the way this game is built, building damage is worthless unless you can instantly kill your enemy or survive/evade CC. Tanks/Champions that can build tanky don't have this weakness because they can take burst, CC, or what have you and keep dishing out damage. This happens all while the Assassin will have to wait on cooldowns or run out of mana.