Hhmmm....Well, I'd think it isn't more popular due to the cost of the ammunition and that its bottleneck case is not as convenient to reload for some folks.
It terms of not being able to find it at Wal-Mart. It's always in my Wal-Mart. Even during the late, great ammunition shortages---.357sig was right there on the shelf. [Another testament of sorts to it's lack of popularity, I guess.] Interestingly enough, while I couldn't find 9mm, .45acp, .380acp, .357mag, & .38sp---.40s&W was also on the shelves (i.e., along with .357sig). [Go figure