Drafting provides vastly less help on climbs than on flat ground because of the low speeds. And when going down the opposite side, being a skilled descender can often allow a lone rider to out-pace a group without really putting in more effort. So it's often possible for small groups and even solo attacks to simply get away from a much bigger group that wants to chase them down and create time gaps.

Furthermore, because uphill riding causes gravitational drag to dominate rather than aerodynamic drag, increasing your effort by a certain amount creates a bigger relative difference in speed. On a flat road, if I double my power output, I might end up going only 30% faster or so; on a 20% uphill gradient, if I double my power output, I'll end up going nearly twice as fast. Compared with flat-ground riding, this amplifies the benefits of an exemplary effort: pedaling 5% harder than a fellow GC contender on a 30-minute climb to a mountaintop finish produces a bigger time gap than pedaling 5% harder than them on a 30-minute flat-ground individual time trial.