I think that because Ekko's current damage isn't high-burst as is. It requires commitment to triggering his passive through chained spells and autos, plus utilizing slows mobility and positioning to ensure that (A) both hits of his Q land on a target and (B) they remain confined within his stun-sphere's landing zone long enough to activate the stun+shield. Even his ultimate requires maneuvering control and planning (plus some reliance on lack of enemy battlefield awareness) to ensure his enemies are within the clone's path if Ekko wants to use it offensively.
Ekko's damage is combo and positioning reliant and can be countered by constant movement hard CC and/or silences to ensure he doesn't trap you nearby his clone or in his sphere.
Removing a slowing skill will greatly reduce Ekko's damage by making it harder for him to permaslow you and lock you down.