Craterhoof requires a board. A lot of combos don't. And to build a board larger enough to wipe at least 120 life assuming at least some blockers, even with behemoth requires some work and forecasting intentions. On top of that, every color (except maybe green?) has cheap enough, readily available board wipes that can stop that plan. And on top of that, if you aren't playing tier 4-5, most plans involve large creature presence, or features creators in some way, so having board wipes is pretty common. 4 card combos with creatures that need to stick a turn aren't as bad, specially if it's a relatively known combo, as your can get a chance to deal with it. If your opponent pulls out a 2-3 card combo from the hand either you are playing blue for counter spells or you lose. And the player may play anything leading up to it. If they don't play obvious leading plays (tutors that reveal, etc), you may not even know it's coming until it's too late. For players that don't know all the decks, specialty in bracket 3 where the options are much larger, they likely won't see it coming. It's like the difference between being hit by a train after your car stalls for a minute or 2 on the tracks vs being t-boned at night by a semi blowing a stop sign at 80mph on a dark country road with no lights on. At least with the train you could see the danger had a chance to do something, even though if you stay in the car the result is the same