Also mapquest was a service with lots of users, but google's maps were such a better offering that people switched overnight to it.
You could argue that google hasn't made enough of compelling service for people to bother switching from yahoo finance to it.
As for email, is a totally different thing. I still use hotmail, mainly b/c it happen to be the first email i signed up with, and many of my old friends have that address, so it is not the easiest thing to switch. (I also do have gmail, and a yahoo email account).
While simple financial info lookup, just as search, are less sticky.
Not to be pedantic, but it didn't - Yahoo didn't have their own search engine for years, only searching of their traditional hierarchical directory and using a 3rd party's search engine, and one of the key reasons Google grew so fast is that they partnered with Yahoo to provide their search results.
Perhaps things have changed this year, with the release of the real-time quotes.