I ran in university, I raced bikes, and I have done triathlons. Each group seems to feel the other groups are elitist and hard to get along with while they think of themselves as easy going and awesome. In my experience, other than a few actual a-holes in each group, most people were very friendly and easy to get along with, down to earth etc., (once you talk to them) in each sport.
That being said, in terms of the time to break through the initial standoffishness, cyclist were the worst. But, largest number of wanna-be insecure pretend elitist losers in each group: triathlon, then cycling, then running. I blame the equipment. The more stuff you have to (or can) buy for any sport (the more money you can spend on equipment and looking the part), the more losers the sport will have. Guys who feel they can buy their way into being good. And, in general, in my experience the more gear a guy has, the bigger a loser he is and often the guy who sucked.
Not universally true, but, if we are here to generalize...