The lava sauce went away with the volcano menu because, although it DID have die hard fans, it just didnt sell the best. I would make like 3 volcano items in about 10 hours. read more
Granted we did have quite a bit more people adding lava sauce to other items, but Taco Bell needed to remove items to make room for new menu items (i believe at the time it was the grillers and the Cantina Bowls/Burritos [now known as Power Bowls/Burritos]) as well as not going to just have a sauce that doesnt come on any menu items, they also took the sauce away to make room for a new sauce, which at the time I believe was the Chipotle sauce. read more
No one can say why it's only promotional except Taco Bell. Their reasoning for this and every other promotional item is that they don't see it being worth adding to the menu. They do plenty of market research on this kind of stuff. read more