There are two reasons that are keeping discs off drop bar bikes:

- Safety. Mountain Bike/Fork manufacturers have recently been through a period of sleepless nights due to stronger disks overcoming the quick release or just plain tearing the dropouts/disk mounts off the fork. Most now have strict limits on disc sizes. Marzocchi have gone to post mount on their big forks to try and solve issues due to the additional braking (breaking!) power of 8" rotors. Disc mounts on carbon forks are not ready for prime time.

- Traction. I run Shimano Deore M535 Hydros with 185mm avid rotors on my MTB and can lock up a 2.5" tyre with one finger. If I run 2" tyres I have to be very careful as it becomes considerably easier to lock things up and I have washed out the front on many occasions. There is no point in running discs with 37mm or god forbid 23mm tyres as you do not have the traction to make use of the braking power that discs afford. Two wheel drifts on drop bars are not fun.