Could it be that your LO is afraid of falling off the bed? I know it can be scary for a child to be higher off the ground after the transition from a toddler bed to a big girl twin bed. Do you have bed rails on to make her feel more secure and safe?
Other than fright causing her to sleep on the floor, I would have to agree with the previous poster...maybe she just likes the harder surface.
Good Luck!!
Angela