I think it's because it's easy to use. Chatters tend to be not the most tech-savvy, but rather normal people like teens.
Well for me, Yahoo is at its best when it comes to its audibles . I really like them and one ore thing that yahoo messenger make its uniqueness is its global chat room wherein you can enter on those different countries chat room and mingled with others.
there is one thing i would like to see for free is pc to phone communications. I think yahoo or msn did this for free like 6 years ago?