A lot of people still use a yahoo email. I suppose when they first got their email, it was supposedly better than hotmail (gmail hasn't even come out yet), and they haven't bothered to change since because, really, it's good enough. Yahoo messenger too, I guess. And probably a lot of people have yahoo as their homepage (for some reason...wonder why!) and never bothered to switch it to something else more sensible. That might explain how yahoo.com is still top 5 on Alexa[1].
You actually bring up a great point about the Yahoo's homepage.
I think I remember a while ago that Yahoo's homepage was automatically set when I bought a new laptop....you know the bundleware crap that laptop makers have set when you buy a pc laptop.
[1]: http://www.alexa.com/siteinfo/yahoo.com