Why Vlad's E costs him his maximum health instead of his current health?

Because every other champion with health costs, other than Soraka, have their costs either in a flat amount or a current amount of their health. Vladimir (and Soraka, but she doesn't use her W to directly trade with opponents like Vlad does) is the only champion with health costs that has this particular feature which makes his sustain usually not enough since he is still killing himself trying to trade with opponents. Not to mention, this is the ability that Vlad uses for waveclearing.

I wouldn't even mind if his E damage was nerfed just to have its cost changed to "10% of his current health" instead of "10% of his maximum health".