Same reason Norah Jones and other people with HUGE mainstream debuts faded. Unless you're a straight up pop star, it's very difficult to make that same magical moment happen again. It's like a perfect storm when those types of albums happen. Sometimes they're reactions/backlash to what's on the radio at that time; sometimes they are actually from a popular genre, but then that genre or style peaks; sometimes they take too long to release a follow up (this is a big one imo) and people move on... etc.
It happened with my own fave tbh, No Doubt released Tragic Kingdom in 1995, it went diamond... but they took too long for their next album. Return of Saturn came out in 2000, when the alternative boom had died down and Britney, Xtina, boy bands, and girl groups were dominating, and perhaps people had forgotten about them. But then No Doubt came back hard with their next album, a more pop-friendly offering, and they smashed. Alanis obviously never did that, though