Part of the problem is that handling worldwide searches with a large variation in admin boundaries, place names, and addresses is a difficult problem. Secondly there are only a few volunteers working on Nominatim in their spare time.

There was some attempt to restrict the results to the current map view, but that might not be active because it confuses others when a small area zoom gives no results.

https://lists.openstreetmap.org/pipermail/talk/2015-December/075201.html