While apathy certainly explains many mediocre instructors, I know for a fact that some outstanding scientists desperately want to be good at teaching but just don't have the magic touch. As a mathematician might put it, knowing the material is a necessary but not a sufficient condition for being able to explain it.

It's not a magic touch, it's a learnable skill.

A skill that's damn hard to learn and takes years of dedication, introspection, and humility, but learnable nonetheless.

So is playing basketball, but we can't all be Earvin Johnson.