Why does a 5e fireball do 8d6 vs 6d6?

@dmdavidblog @JeremyECrawfordWhy does a 5e fireball do 8d6 vs 6d6? Because monsters have more hp? To offset lack of caster-level boost? it was 6d6 for a while, but that was a little too weak as everything fell into place. easy to boost one spell

— Mike Mearls (@mikemearls) December 12, 2015