You first have to look at when the term came into being. It is an old term that the road players use to use. They would refer to someone as a shortstop that was willing to play an unkown and would bet decently.
The road player would not have to come into town and scope out the town for a while to find out who to play, but rather, could just come into town and play the "shortstop" right away, win, and then move on to the next town. The road player wouldn't have to spend a lot of time in an area trying to get a game. Hence, a shortstop.
As you would assume, the shortstop was a very good player, but not quite good enough to take on the road players which often were what we call the top pros today.