[btilly]

That is no accident.

A requirement for clear speech is clear thinking. Very clear thinking is required to be a successful polymath in a technically challenging field.

In order to accomplish in a technical field, such as math, you need to think very "efficiently". It is possible to do this by spending so long coming up with a mental model of a specific area that your thoughts are efficient, but cannot necessarily be communicated to people except when their mental understanding is a reasonable match to your own.

However you cannot do this simultaneously in a great many areas. Therefore a polymath must find ways to construct models that are both simple and efficient. The underlying simplicity of their understanding then becomes evident in how clearly they can explain a precise understanding of whatever they are talking about.

[kornish]

Well phrased. It's no coincidence that Feynman, another famous polymath, was also famously clear and concise.

[kkylin]

I agree with all that, though I also want to add that the converse is not true -- clear thinking is not, by itself, enough for clear exposition. The latter requires investing additional effort, which many people are unwilling or unable to make.