I'd say the deraileur drivetrain works well for road bikes that generally don't deal with agressive riding conditions. That way the advantages, mainly light weight and efficiency, outweigh the shortcomings, such as exposure to elements, which in turn lead to more maintenance work.
And understandably, commuter and "gravity" mountain bikes, where light weight and drivetrain efficiency is not of such importance, are more willing to look into other options which are better protected from environment such as mud, sand, salt, twigs, etc. and physical damage.
Hoarses for courses, as they say.