The strategy is so popular because it appears broken to inexperienced users. The fact that you can boost both of your defenses and heighten your Special Attack with an insanely powerful move to pair, it seems unstoppable. However, it is unviable because it pretty much gets hard walled by Dark-types. If your opponent is smart, they're going to keep their Dark-types alive until Sigilyph is dead (this sort of applies to all sets as well). On top of that, there are other checks that work really well, even after one boost with Cosmic Power, such as Heliolisk and Rhydon. To put it simply, it can be effective if your opponent is dumb and sacks all the obvious checks, but that generally doesn't happen against experienced players.