No reason as far as I'm concerned.
Possibly when the Mk3 prices started at £899 it made it a no-brainer but The Fratello in Shimano 105 Spec is about £400 cheaper than the Mk3 in similar spec.
I was going to buy a Mk3 on Cyclescheme because my friend bought one last year and it pleased with it. I saw the latest Thorn price list and had second thoughts.
On Saturday I collected my new bike, a Condor Fratello. This is it today, at lunch time
Shimano 105 "Black" Triple x 10spd
Mavic Aksium Wheels
Deda Black Rain carbon forks
Brooks B17 Champion
£1250
Too early to say how good it is but it felt lovely today on it's first proper ride.
Edit - The Thorn no quibble guarantee is a good reason.