Carbon Rims?

OK, I admit ignorance on this (a dangerous thing to do on RBR...).

VeloNews live Vuelta coverage says, Stage 9: "4:35 p.m. Up front, the lead
group has dropped Cardenas on the descent ... he is being extra cautious on
the wet road, apparently reasoning that you can't win if you finish in the
ambulance. Our man on the scene, Charles Pelkey, notes that he's using
carbon rims, which may be a factor."

Why would carbon handle less well in the wet than metal rims?

Al