Both the Asset Registry and Asset Manager look for all subclasses of UBlueprintCore and use that to search for blueprint classes and construct an inheritance hierarchy. This always works in editor builds (after [Link Removed] was fixed). This also works fine for most children of UBlueprint because even though that class is not loaded in cooked builds the native class does exist.
However, UBaseWidgetBlueprint (and UWidgetBlueprint) are editor only classes so it is not available when the asset registry tries to construct the BP class hierarchy that is then used by functions like GetDerivedClassNames. This means that widget blueprint assets are not added to the hierarchy and cannot be queried or used as Primary Assets in the asset manager.
From a UDN report, here's a programmer-only way to see the core problem:
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-160694 in the post.