Don't know about third world, but compared to Europe and Asia, large homes with space for two machines may be more common in the US?
Simply combining the two machines into one unit increases it's chance of failure even without changing the reliability of the washer or dryer. You don't consider your dryer broken if your washing machine breaks, but you consider your washer/dryer broken if either the washer or dryer function breaks.
I would guess that perhaps a major reason is mindset, Americans were always used to two machines, while most Asians have only ever seen and used a combined machine.
Think of it this way, will people who are used to combined machine be likely to switch to seperate machines? My guess is they wont.
In third world I think only combined machines are available in market usually.