Because he was signed.
And if he is having the kind of year he had last year, and Rizzo gets hurt and goes on the DL, I suspect that he would be called up and given the chance to fill in. Probably won't do very well, but he is still the most likely replacement at this point, in my opinion.
Circumstances can change. If Stewart is back to his form of a couple of years ago, they might well put him at first, and put Valbuena at third. But if both Stewart and Valbuena is struggling, as is quite possible, Nelson is probably the guy.