I think the thing about triathletes that irks most people is that their sport is one of those niche sports where there is not much depth and you kind of have to be rich to compete. It is the opposite of running: cheap, simple and anyone can do it. To be a top runner, you have to be faster than a huge pool of competitors. To be a top triathlete, you have to be pretty good at three different sports and have the expendable income to purchase high end gear. We, as runners, feel that we have more to brag about with our accomplishments - after all, our sport is more competitive and doesn't depend heavily on the size of our wallets.
Triathletes interpret the miniscule participation in their sport to mean that other people are too weak to compete with them. They do not stop to consider that it could be due instead to a lack of interest. It is the same with ultra-marathoners. You, me and your cousin could probably walk-jog 100 miles, but we have better things to do with our time. We don't do ultra-marathons because we don't want to, not because we can't.
This is an important distinction and I think there would be much less animosity between triathletes/ultra-runners and normal runners if the former group understood this a little better.
Anyway, just my two cents.