It's easier on your body
During a marathon, runners often experience something that's been dubbed "hitting the wall" (already sounds spectacular), which could also be described as "the moment you realize the first person to do one of these fell down dead."
Hitting the wall is when most of your body's glycogen stores -- your main source of energy -- have been depleted. Your body starts to break down fat and protein for energy, but this takes longer. Your blood sugar is low, and you can start to feel dizzy, shaky, and confused. This tends to happen for runners around mile 20, with another 6.2 miles to go. That's a full 10K, and your body's out of energy.
Screw that! In a half-marathon, you never even get to that 20-mile mark. Catastrophe averted. That's not to say that a half is easy peasy -- you still need to prepare for this stuff, because you risk ankle sprains, muscle strains, dehydration, overhydration, and loads of other ways that your body can bug out on you if you don't know what you're doing or don't have the proper medical clearance. But the destruction that can befall even healthy, prepared people during the latter half of the marathon just doesn't happen, because that's a hell of a lot of miles, and your body wasn't designed to run them.