All versions of Zope depend heavily on the Zope Object Database (ZODB). Because ZODB is less a database and more a persistent object store (it doesn’t possess a query language; Python is its query language), it has been necessary to create indexing and searching facilities for data stored in ZODB.
The first iteration of searching and indexing for ZODB-based applications (at least post-Principia, which had something named Tabula, which I never actually used) was the ZCatalog. The ZCatalog was entirely tied to Zope2, and still remains in heavy use today within Zope 2 applications such as Plone.
The second iteration was zope.app.catalog, which was a ZCatalog do-over for Zope 3 applications.
Neither of these searching and indexing packages are particularly easy to use outside of a Zope application. Each makes various assumptions about the content objects that need indexing or the environment that aren’t appropriate for arbitrary applications. For instance, ZCatalog wants objects you want to catalog to have a getPhysicalPath method which returns a “path”. An instance of zope.app.catalog makes the assumption that that it’s located within a Zope 3 “site” object within a ZODB, and assumes that you want query result sets to be sets of Python references to the original object you indexed. In other words, these packages assume too much to be maximally useful outside the context in which they were developed. Repoze is a project which has as a stated goal making it easier for non-Zope Python developers to use Zope technologies outside Zope, so this seemed like a natural thing to do under the Repoze flag.