User blog comment:Lokiwg/Dear Ekko players/@comment-24420226-20150627212621

At first it was just because Luden'd Echo overtuned - just like Lichbane was. In comparaison, it was similar to having 4 lichbane procs at once on 4 different targets, deliverable via any long-range spell (Ekko's Q). The movement speed bonus is also quite significant, outshined only by Zephyr for percentage movement speed, which is huge for a champion that will spend time roaming in the midgame. Plus, you gain stacks whenever you use your E to dash/blink, or your Ult to rewind, making it a lot easier than you think to build stacks back up. Plus, that additional lichbane proc might not matter, since your intended assassination target may already be dead after your first combo.

For a roaming assassin who's only goal is to blow up an enemy carry as fast as possible, Luden's Echo is currently the ideal 120 AP item to purchase first; Death Cap only makes sense once you have two or three other AP items to stack with. Ekko himself has less need for an Hourglass than other such assassins since he can (theoretically) rewind to safety - though Hourglass is still a great item that works well on any AP caster.

However, the best option is always to have both Luden's and Lichbane ;)