pads wear. mech disc setups do not self-adjust in the way hydraulic systems do. essentially the pads require longer travel as they wear before initial bite (+less power).

all mech systems that exist today (that i know of) have their adjustment at the caliper, so it is not just a matter of reaching down during a race/ride.

it is especially noticeable when things are wet and/or dirty, so less noticeable on the road (but probably an issue in the very conditions people most pine for disc brakes on the road).

go try a bb7 setup vs a hydraulic setup for 5 laps on a cross race and you can feel the difference. it's not subtle.