Considering you do not know her history...coming from a Shelter, it's quite possible she has made a routine of sleeping on floors especially if her prior owners did not provide her with a bed of her own.
My rescue dogs were all fickled at first and slept here and there but, after awhile and a little persuasion with treats hidden in their beds they all have learned to l
ve their beds.
All of my girls still like to do their own things but, come winter time it's always snuggle time in bed with Momma!