Haven't a clue as to how clematis got that name dream gardener. Perhaps someone here has an idea. However, I would suggest you contact Edith Malek, the president of the American Clematis Society, since she is always using the term and may have some insight as to that. You can contact the American Clematis Society at info@clematis.org.

I also have no idea where or when that tag became associated with clematis, but they have been referred to in that manner for many years. I would assume that is because of all the vines, clematis offer some of the showiest blooms, the greatest variety and the most adaptability to various locations. They have been cultivated and hybridized longer than any other flowering vine as well

Don't know about the name, offhand, but the Illustrated Encyclopedia of Clematis has an excellent history of clematis in it, including some wonderful old photos.

You could also ask at Garden Buddies forum.

I'm sure someone will know.

http://www.gardenbuddies.com/forums/clematis/