Well, I wouldn't say the stable failed, really. It did exactly what it was supposed to do; it had mid card feuds for a little while, and disappeared. They got mid card to main event feuds with The Undertaker, Kane, Booker T, RVD, and co. I wouldn't say it was a failed group, so much as it didn't live up to your expectations.
It gets forgotten because, it was a mid card stable. That, plus it was a redundant group, that's been done over and over in the history of wrestling. It seems like we get foreign groups who hate Americans every two years on television. So the group isn't forgotten, it just doesn't stand out compared to the hundreds of similar groups that play the exact same role.
As for a forgotten group over time, I'll go with the Ministry. History will never allude to this group, as they were pretty damn Satanic over time, and admittedly, that's probably never going to air on WWE Television ever again. But at the time, they played a better cult role than the Straight Edge Society does. The things they did were shocking, to the point of making you uncomfortable, which was the exact hope of the group. They hoped to scare the everlasting shit out of you, and that's exactly what they did, at least to me. I don't care who you are, watching your favorite facebeing crucified, and potentially burned to a crisp is something that will frighten you to death. Sure, all of the wrestlers weren't great. Ok, I'll be more frank; Mideon and Viscera were the absolute shits, but they were good for what they were brought in for. Other than that, you had The Brood, who also worked with them, The Acolytes, and The Undertaker himself. I feel that a group like this, at least, was a little original, and downright freaky.
So yeah, The Undertaker scares us shitless. What else is new?