Your genaralising. I don't think all jugglers are smart.Need to see evidence (Period)
Read up about Howard Gardner's Multiple Intelligences. Jugglers would generally be high in bodily-kinesthetic intelligence.Logical-mathematical intelligence (number/reasoning)
Well think about it, how many people in their hobbies, or professions keep learning at the same rate we do? Just the amount of information we process to learn a new trick must be tremendous!
Once a musician knows HOW to play their instrument, it's just memorizing which order to play them in to make music.
How about composing music, or creating a new way of playing an instrument? Hell, I myself am not musical person at all (in way of playing music) but I think music can be bit more complex than just memorizing order of notes. But this is bit off the topic..