On Sunday, October 22, 2017 at 9:36:31 PM UTC-4, AMuzi wrote:
Snipped
Thanks.
In practice, large diameter discs seem to have less pressure
on the fork blade mounts and builders drop the material wall
thickness with big rotors. Consider tandem ten inch discs,
versus the trendy little 140m 'road' discs, where wheel size
is constant.

Yes, we've previously discussed the advantages of mounting a
disc caliper on the front of the fork. IMHO manufacturers
don't want to dance with troubles from fastener or caliper
mount failure, preferring to load the caliper down against
the blade rather than pulling away from it (home and XMart
installation being what it is).

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

Could also be that mounting behind the front fork leg gives more protection to the unit. After all, the disc brakes were put on MTB bikes before they went onto road or touring bikes and MTB are hitting or coming very close to all sorts of obstructions.

Cheers