The Peace prize is awarded by Norwegians, by some request in Alfred Nobel's will. The rest of the awards are decided by Swedish committees.
They take every chance they get to get foreign A-list celebrities to come to Norway.
1) They can get top notch music artists to perform for free at the ceremony.
2) They get Hollywood superstars (e.g. Scarlet Johansson) to host the event.
3) Third and final sell-out is giving the price to someone famous.
This kind of takes some of the prestige away from the proper scientific Nobel prizes.