Lux misses Thunderlord's, a lot. E>AA>P was and still is a core part of laning with her, since it's the easiest and quickest way to get a positive trade. But in the olden days, Lux passive counted as that third hit, which caused Thunderlord's to go off. Electrocute, for some unknown reason, doesn't work like that.
So, easy fix, make Lux's passive damage come in .125 seconds after the auto damage, thus counting it as an entirely separate attack and allowing Electrocute/Phase Rush to go off. Given that Electrocute is already "balanced" relative to Thunderlord's by it's substantially longer cooldown, making it also harder to activate seems unnecessary to me.