It is by his style.

By that he makes what is easy difficult.

The reader does not get at what he means as he gets at what Homer, Dante, and Shakespeare mean.

Dante and Shakespeare are often difficult through the depth and difficulty of their matter; they are not difficult, except Shakespeare when he was learning his art, by obscurity or carelessness of style.

But Browning is difficult not by his thoughts, but by his expression of them.