User blog comment:PurpleWii/The Evolution of the Tank archetype./@comment-1330314-20160425182609/@comment-24420226-20160426020121

Percent health based on the enemy's HP is just a way to ensure a champion does a base-line level damage - it's something they can fall back to even if they can't build a lot of damage items (because they fall behind or they simply aren't expected to). This damage will inflict the same proportion of an enemy's health regardless of how much health the actully have, allowing them to keep up with snowballing tanks and whatnot.

Mathematically, percentage health tends to be more threatening to squishier carries than durable tanks because tanks tend to build lots of resistances alongside that health, whereas carries will ony have maybe one or two items with smaller amounts of armor or magic resistance. So while the attack will always deliver the same fraction of an enemy's health regardless of how tanky they are, tanks with resistances will be able to better reduce the fraction of health that they lose mitigate through resistances that carries usually lack.