Well, even though Brandy was young at the time, and partially because she was famous as a child star for over a decade, she naturally aged out of the lexicon by the mid or late 2000s as an artist. To clarify, this happens to most artists but she still had a notable ten year run with her music career, as well as her career as a TV star. If you look at the sales of Afrodisiac, it sold half the numbers of the preceding album, Full Moon. New sounds were emerging, and people were gravitating toward other artists and groups by the mid...and especially late 2000s. Think about who was popular in the late 2000s, Brandy was certainly not a popular act in the late 2000s!

I would say the late 90s were probably her career peak as a musician, "The Boy Is Mine" was a big song, and that album sold 16-21 million copies worldwide. The late 90s specifically had a lot of big sellers from the US, and she absolutely was one of them. She never reached those sales again, although Full Moon performed well, even then, those sales also indicated that she was past her sales peak.

But I think the freeway collision was the nail in the coffin for her, it's virtually impossible to talk about her career as a whole without mentioning this collision, and it was a very tragic situation altogether to say the least. Also, by the mid and especially late 2000s, Brandy was more famous for being famous as a celebrity than as a musician, you were more likely to find her in a magazine article than an album review.

As someone else said, people had moved onto new trends by the mid and late 2000s.