I guess because she's been famous for a long period of time many people know her, even if she isn't the hottest thing any more. I think it's the same with Aya Ueto. She also had her prime some years ago, but she still has tons of CMs. Companies know that she'll appeal to the public and does a good job. (Well, at least I suppose that it somehow works this way).
And she still does put out good music. Hopefully she'll do the same as Namie and has a second prime
.