Why doesn't Vayne have %bonushealth true damage?

Like, seriously, she's most commonly described as a late-game tank shredder. However, with the idea of %maxhealth true damage(with flat), she shreds tanks and squishies alike, and after she finishes her BotRK she's on a pretty even playing field with most ADC's(minus maybe strong burst ones like Draven, but even then sometimes).

However, with %bonushealth, that would make it so not only does it not work too well until the mid-late game, but it also doesn't shred squishies just as well as tanks. Plus, the flat true damage on it is pretty much enough to make it semi-viable early game anyway so its not a totally useless spell you don't get until you can't stop upgrading your Q or whatever.

It would just make a lot more sense gameplay wise, seeing as what her role is supposed to be.