Not much of a rise in popularity as much as something people are into at the moment. Mumford and Sons is the only top 40 folksy band out there, the rest are overlooked. Not a great recipe for prolonged popularity, especially considering Mumford aren't exactly a top notch act.
Prolonged radio popularity is a good point. The Lumineers are on their way I think also. However I don't think folk will ever really catch on with the radio crowd but is very prevalent in the indie scene. More so because I don't think it would be classified, subclassified or whatever you might call it in with indie. So the rise of folk bands just under mainstream is a pretty good step.