While I'm not technically French, the answer to CC's question is that her music was written for young teenagers or kids, and so her fans back then have grown up and grown out of her songs after 20 years apart from for nostalgia purposes. Young people today listen to Angèle, OrelSan and Stromae. Mylène fans are more of a "cult" like thing than mainstream.
I still can't understand why the idea that French can't support catchy melodies is repeated here when it is easily disproved in less than 4 minutes:
French music > English music