It isn't so much about sturdiness as a Thorn fork is hella strong but is also a great riding fork... disc forks have to be extremely stiff to withstand braking forces and the disc tabs and brazing also affect the steel.

It is a great point of pride to be able build a strong fork that also improves the overall ride quality of the bicycle.

The Surly 1x1 is tandem worthy and bikes fitted with these rely on some high volume tyres to provide a quality ride and to dampen road buzz... my hybrid runs a monster of a steel fork that is stiff as hell and the bike cannot run on narrow high psi tyres if you want any kind of ride quality.

Get to riding 100,s and 100's of bikes and you will be able to see the difference between a so so fork and one that has been carefully and skillfully made... the Carlton made forks on many Raleighs are wondrous as you can run very high pressure tyres and still get a beautiful ride.

The fork on my handbuilt Ron Cooper is a masterpiece in this respect and is by no means weak and the frame is stiff and responsive... but with 130 psi tyres the ride is still amazing. If a disc tab was added much of what makes that fork so good would be lost.