Someone feel free to correct me, but Sheen (and it's upgrades) are force multipliers that aren't universally applicable.
Getting
on
and
will benefit both equally, because it enhances all of their abilities equally and results in the same damage output.
Getting
on
and
doesn't benefit them both equally. While their abilities benefit equally, the play pattern differences mean that Ekko gets far more out of the purchase than Xerath, simply because he can use the passive more often/more reliably.
Now with that in mind, add to that the strengths and weaknesses of different champions that can reasonably use
. It's almost impossible to have an additional passive for the item's upgrade that's both desirable and doesn't push someone's strengths too far or mitigate their weaknesses too much.
The common abuser is
, because he's able to apply the on-hit faster and from further away than any other champion (being able to build AP or AD helps too). His weaknesses include limited waveclear, no CC, and being squishy, while he's normally picked for his range, mobility, and damage. If a new sheen item provides more waveclear, CC, defense, or damage, (hint: it will provide one of those) chances are he'll give it a try. Because he can use Sheen to a much greater extend than someone who has to be right on top of you to use it (like
), he gets to apply his force multiplier more frequently, thus abusing/benefiting from it more. In many ways, this is a similar parallel to the Ekko/Xerath example, since Ezreal's play pattern lets him activate the passive far more often than Nasus could.