Because pound for pound mules are stonger and more surefooted than horses...but I believe they use burros and not mules btw.

I think it is because mules are more hardy and sure footed the a horse...but i could be wrong im only guessing:-)

They have more endurance and are smaller

Horses are skittery. They start and bolt easily. Those tendencies have been bred out of the sure-footed mules. I believe there has only been one major mule accident in the canyon in the past two centuries. Signed: done the trip in 132 degree weather.

Mules are steadier on their feet and have better balance than horses, they inherit that from their donkey dads. Mules are 'daintier' steppers, they seem to know or even see where they are putting their feet, they can usually put their rear foot in the same place their front foot was, and often in the same place the mule in front put its feet. Mules are steadier than horses, they don't get all excited and unmanageable about stuff in general. If a horse gets all scared about the steepness, narrowness, or the dizzying height of the trail it will often just throw a conniption fit and twist, shy, or even buck; a mule just stops, a mule is smart enough to know to remain calm in situations like that, or it too could go over the side.. Mules are smarter than horses when it comes to stuff like that. Just like when you try to make a pull or carry too heavy a load, they aren't just being stubborn,they know they could try to do it and probably succeed, but they might bust a gut or wear themselves out prematurly. Even if a horse remains calm it will sometimes go right ahead and try a path that the stupid human wants them to, even if the horse itself knows better. Horses are always suckin up to humans. Mules know they are smarter than humans about stuff like that, " No way, boss, if you wanta go down that slippery slope go right ahead, but I'm gonna stay here."

i read that mules and burros will not go down an unsafe path. They will refuse to move.

Not sure if you're still looking for answers, but I just returned from a vacation that included the mule trip into the Grand Canyon (26-27 Sept 2008). Re why they use mules, our guide said mules are smarter than horses, and that, where a horse might panic, flail around and slide over the edge, a mule will think "sh**, I'm not going there!" and save himself (and rider) from toppling into the canyon. :) Trip was great, BTW - I'm ready to go again! Cost was about $420, which included staying one night at the bottom. There's also a day trip that goes partway down ($150), and a 2-days-at-the-bottom trip ($500-600). These are all from the South Rim. Here's a link for more info about these rides and those from the North Rim. http://www.nps.gov/grca/planyourvisit/mule_trips.htm

Mules and Donkeys cannot be moved if they don't want to be moved. They stick their toes in and freeze. This is true of times when they are frightened as well. Where a horse might panic and go galloping and bucking away from something, the mule will just refuse to move.