Thru-hiking is hard.
Yes, I know that is an obvious statement to make. I doubt many people believe walking hundreds or thousands of miles is easy. I'm not saying this to suggest that when I hike I'm doing something particularly extraordinary. Many people can do what I'm doing. There are several hundred of us on the CDT right now, and thousands more are on many other long trails.
Many aspects of thru-hiking make this activity difficult, and each one can be a reason someone fails to finish their hike. None of these potential reasons for quitting is easy to overcome, but one is particularly random and troublesome. That is injury.