"The main reason is that most don't want to run ultras. It's appealing to only a small percentage of runners."
I disagree. When you run track and cross country in high school, both are options when you go to high school. And when you turn on the TV, you can watch track and marathon races. None of this infrastructure exists for trail and ultra running. Kids don't know its an option unless they live in a trail town.
" WS 100 seems like a miserable experience"
Western States is 100 miles. There are far more 50k races in the world, a nice round number, only a few miles further than the arbitrary 26.2 of the marathon. In a world that is largely metric, the marathon is the most popular distance due to a simple trick of historical luck.
"and isn't there a high risk of injury in training and racing ultras?"
The typical ultra is run on trail, a softer surface than man-made concrete and asphalt. And if the comments on Let's Run are to believed, all ultra running is slow. Shouldn't the injury risk be lower if we're all running slow?