I assume that by "ATPG" coverage, you mean "stuck-at" fault coverage, and if so, the answer is fairly straight-forward: stuck-at coverage detects a more catastrophic fault, whereas the transition delay fault must be able to detect both a slow-to-rise or slow-to-fall - which are both more difficult to sensitize and propagate, because the tool must set up a transition, not just a static faulty value.
If you have more questions, you might ask over at DFT Forum, as there are more DFT types hanging out over there...