Then you'd have traded a 20-some second universal mitigation CD for a one-time skill that works only against a single tankbuster and only assuming an auto-attack doesn't hit just before it, just to have further excess HP after the tankbuster, at the cost of total mitigation and applicability. You'd have nerfed PLD greatly in some 95% of circumstances.
Even at 60% you would still just be intentionally imbalancing PLD, making it overtuned for raids and undertuned for dungeons. So... why? Percentile mitigation for duration and/or until a given absorption cap are just far superior choices than affecting only one attack.
