Two reasons really.
Overheating when drag braking on long descents with resultant fade when you need the braking at the bottom of the hill (Robin Thorn has a good rant about this too on his website, especially for tandems).
Also, lack of comfort at the front because forks have to be made stronger.
It’s in the latest magazine.
My experience is that the discs which work so well off-road in Morzine can fade on the long fast road descent from Avoriaz. I am ‘big-boned’, though.