Well, the two item rule is to conserve overhead space (I would assume, at least, though maybe also to speed up boarding), and a suit doesn't take up overhead space.

I've carried on coats folded over my arm before many times - and that has never counted as a carry on. And that often ends up on top of my roller in the overhead bin. But adding a hanger to the mix (and removing it from the overheads) suddenly makes this a problem... Just seems strange to me.