I'm guessing it's popular amongst people who install software that in turn installs a yahoo search toolbar on them that they cannot figure out how to turn off/don't bother to.
http://en.wikipedia.org/wiki/Yahoo!_Search
Apparently Yahoo is the "2nd largest search directory on the web by query volume, at 6.42%, after its competitor Google at 85.35% and before Baidu at 3.67%".
- First of all, wow, what a landslide! - Secondly, Yahoo still has a pretty big slice of that pie relative to everyone else. - Super surprising that Google owns 85% of the search market, yet is still behind Yahoo on traffic :/