I think that's exactly it too. I believe it's more of a "it's hard to break new ground" problem with romance as I think its not as broad of a genre as shonen or seinen. With romance it's main focus will always be romance, and there is really only so much you can do with that because if you add too much of anything else, you'll probably end up writing in more of the direction of another genre.
And when you first mentioned romance, I honestly thought of Clannad first. It is absolutely a work of art. I watched the anime first, and then I played the visual novel and I tell you what, they did a hell of a job on making that a classy series that stayed away from feeling like a harem or ecchi.