How come Witch Bolt’s sustained damage doesn’t increase with higher spell slots?

@Boxvoko @JeremyECrawford How come Witch Bolt’s sustained damage doesn’t increase with higher spell slots? It feels a bit weak to me?
I believe because it stacks up too quickly – usually, damage buffs are one or two dice total

— Mike Mearls (@mikemearls) November 20, 2015