Musicians have been writing music scores for centuries. The computer records about all those music scores are sometimes incomplete. For example, in Search It the record for Anthology of piano classics: twenty-eight selected compositions, published 1904, has no description field. Therefore, there is no text to search.
WorldCat collects records from thousands of libraries. One of them had a record with more information - the composer names, the titles, and the keys. Therefore, there is text to search.
For music scores, you will often need to use Worldcat. Use the Advanced Search to get the required options.
WorldCat refers to filters as limits. You can limit the initial search to scores by putting a checkmark next to Musical Scores.
If you made a search but did not set the limit to musical score, you can still get just scores. On the results page they have tabs that let you look at specific types of material.