Well... that's a very good point Peter. The reason I think is because you included "sheet music" in the search; I don't think there is any mention of "sheet music" anywhere on IMSLP... try using "score" instead (I searched for "free score IMSLP" and the IMSLP main page was the 5th result)

The reason I never used the phrase "sheet music" is that I thought it was not entirely correct; in conservatories they never use "sheet music", but rather "scores", because saying "sheet music" has a faint connotation of popular music.
However, I think what you're pointing out is very valid, since indeed many people use the phrase "sheet music" as a synonym for scores, and so I'll try to add a meta tag to index.php which includes "sheet music".
Sphemusator: Not really... but all the major search engines (Google, Yahoo, etc.) has found the site and indexed all the pages
