Cinncinatus was special in 2 ways:

• First he renounces his power as dictator as soon as it wasn't needed anymore. He was called to duty, came and did it, then left once the situation was resolved. The fact that he may have done that twice (458 BC we're sure, 439 BC not so much). Made him some kind of legendary figures.

• And that's my second point, his story was used as a legendary exemple, as the kind of heroic figures you should follow. His image, as a warrior/farmer ready to come save his country, then resigned when not needed was emphasized through the ages, as one of more or less "perfect" leader. Moreover his story was one that inspired great historical figures, most notably George Washington who seemed to have tried to emulate the same story around him (or the similarities were emphasizes afterhand).

Being a man called to action, doing what he could accomplish to his country, then leaving power when he could have taken it with ease, in order to go back to a simple farmer life. If you don't precise who you are talking about this story could apply to both Cinncinatus and Washington.

He is a legendary partly because of his action, "defender of the roman republic who didn't took power when he could have kept it", and partly because his story inspired other leaders throughout history.

I would add that every "legendary" figures in history are so famous because other well known/legendary figures were inspired by them. Part of why Alexander is so well known is the fact that both Ceasar and Napoleon were trying to be like him (his life is also a very good reason, i'm not saying it's the only factor, but it is one we shouldn't discard). The story of Cinncinatus, by inspiring other important figures in history also became legendary because of them.