Sorry, cannot recall where I got the info, believe it was so as to not cause resource damage if a bear worked the Ursack trying to get at the contents.  Would a bear then carry off the bag?  :). I’ve never had an issue with a bear, more trouble with deer.  It’s been over 15 years since that trip.  The Ursack I have isn’t approved anymore as far as I know, so have been using my black can or if in an area with no set manufacture can designation, use my Wild Ideas can since it is much lighter.  I only use a can when required.  Has been over 50 years of bping.
The couple trips into Glacier NP, where required, used a bag hung on the cables.