I think you'd find some evidence for historical poverty being one factor of several in cultural practices of bed sharing. Japan would be a notable exception (plenty of co-sleeping, unremarkable poverty rate).

I also don't think it has become much less common in communities where poverty levels have fallen. Anecdotally, my SO is part of a community originally displaced from India and now settled all over the world, where members have disproportionately become very wealthy. Bed sharing is absolutely the norm in those households, much to my distress when we visit her family!