Why hasn't John Hiatt really attained commercial success as a singer?

I think at least part of it is that he writes somewhat intelligent songs that, while excellent, aren't really what people expect, or have odd analogies or turn of phrase in them that require some thought.

If you consider some of his more popular songs (either sung by him or different artists), they tend to be the fairly straightforward ones. But a lot of his songs aren't like this and, as a result, don't get a lot of radio play I imagine. Then, you get a bit of a cycle where more popular musicians sing his stuff because they're known and he's not, but because the song is being played, the popular version gets the radio time and there's no need for his version to be played, so his popular songs are always being played by other artists, the odder songs aren't being played at all, and there's no need to turn to the Hiatt version since there's a more popular one available.

I also suspect some of his more recent turns into blues and more folky styles are even less radio friendly, although I personally love them just as much.