Probably because it's a semi-amusing, ridiculous thing that we had to see every time we started up a Wii game. I don't think those other things have much of a level of humor behind them.And why Reckless Safety Man of all things? Why not AR Guy? Or the eShop bag?
I think trying to break down the logic behind a joke/meme is a pretty fruitless endeavor. I don't imagine there are a whole lot of people sitting there crossing their fingers for Reckless Safety Man and counting down the days until the ballot ends in hopes that their request will get in the game. It's more likely just really amusing to them and they don't care about the ballot or DLC enough to have anything else they want to vote for.