Because the image locations in the database are keyed to the serial number of the hard disk. So if you restore that database and images to another disk after a disk failure, the database doesn’t recognize the location of your images because the serial numbers don’t match and all edits, keywords, project associations are lost. It an absolutely abhorrent design that, in the absence of a disk in a system with the same serial number, the program can’t do a basic tree search to find the images and relink them. Edits and keywords can be re-imported from the DOP sidecars but the project settings cannot.
So you’re screwed with respect to projects.
All the gory details are here (note this was for a change in the low level structure of an existing disk but a replacement disk would suffer the same fate, even if you used a cloning utility to make the copy.)