I think ultra runners are a lot more flexible with their regard to distance (and terrain).

I agree and think triathletes are far more concerned with their finishing times which is why there is so much demand for the flat courses. In ultra-running, so much depends on the course that the time is far less relevant than just finishing or if you're in the top tier, your placing.

Ultra-runners as a whole are far less interested in having mass exposure or big crowds. The solitary draws them to the sport in the first place and they don't want the aid stations every few miles with crowds cheering them on.

It's also easier to be in the middle of nowhere, no issue of drowning or having a mechanical. If you're not fit enough to run, you can walk.