The expression "walking around with your head in the clouds" is used to describe a person lost in a perpetual fog of revelry. Such people, while usually harmless, tend to be unreliable.
Clouds also bring tidings of bad luck. Think of Pigpen from the "Peanuts" comic strip, with his black cloud lingering ever-presently overhead.
So perhaps "cloud" isn't the best word to describe a concept that some believe is on its way to changing desktop computing as we know it.