In reply to Rog Wilko:
Because seeing where you are going all the time is not a key priority on a closed road, but aero is. Full cone-head helmets have largely disappeared because the rounder aero helmets were better in typical time trial use, and so the penalty for dropping your head is less than it was. If you had belay glasses, you might be able to see better if your head was down (with an aero penalty), but you'd see bugger all when you sat up to take bends, when you needed to see better.