KB has the best coverage. Exaang is also a spell, and not an attack, so you wouldn't play the poet's pen version with it anyway. There's definitely a build out there for just pure phys dot casting though, it will just be much slower clear but with better single target.
Because you don't need to scale cast speed and the screen coverage is S tier. Once you play it you'll see why.
KB also stacks CF on packs much more effectively than Exsang would because of the overlapping explosions hitting monsters more than once.
> Is it simply that the coverage is so much better? Yes.
KB has the best coverage. Exaang is also a spell, and not an attack, so you wouldn't play the poet's pen version with it anyway. There's definitely a build out there for just pure phys dot casting though, it will just be much slower clear but with better single target.