Here's the thing

maybe she keeps stealing the covers, did you think of that?

When the store became main stream they had to have separate beds because at that time only lower class people had and shared the same bed. So by having separate beds the bears seemed like a little family that was middle class. The reason most husband and wife's had separate beds was they were done with having children so a shared bed was no longer needed.

Maybe mama bear had leg cramps at night, or restless leg syndrome and didn't want to disturb papa bear.