Why were labels in iPhoto not transferred when photos were imported to Photos for Mac?

This defines how metadata is transferred - How Photos handles content and metadata from iPhoto and Aperture - Apple Support

However there is no field in iPhoto or Photos named "Label" - what exactly are you talking about (there are descriptions, keywords, file names, titles, locations, faces, etc - but no "labels")

and there is no importing involved - you migrate iPhoto libraries to Photos - do not import anything

LN

Photos does not display all metadata overlayed on the thumbnails like iPhoto or Aperture did.

There are no rating stars and no flag, just the favorite heart. Some metadata can be shown with the thumbnails, if you enable the metadata overlays in the View menu in Photos: View > Metadata. Check all options you want to see.

iPhoto used to show the filename as a default title below the thumbnails. You can see the filename in the info panel, but Photos does not treat the filename as a title. If you want to see it below the thumbnail you will have to copy it to the title field. You can use a script to do that: Script: Batch Changing the Titles to the Filename w/Extension