Actually I think the round is fairly popular. It trails behind the 9mm, 40 S&W and the 45 acp, but it does have a base of support and is used by several law enforcement agencies which always guarantees some popularity.
Why it isn't more popular is that it does not give you all that much more than other rounds. Other rounds penetrate auto glass and bodies fairly easily. Some loads for the .357 Sig will penetrate steel plate better than some other semi rounds see the link below). But there is a trade off for that.