This is a good question. From the mid-pack perspective, I see a lot of focus on marathon training as a goal and a business, so it seems like many get pulled into long distance running because there are peripheral benefits to getting to be a part of that experience. There are not nearly as many sprint opportunities, so the shortest races around are really middle distance races. My best friend is great at the 200 but gets to actually race that about once a year at most. That is his favorite distance and his talent, but the opportunities are so few that he ends up racing long or middle distance instead and gets frustrated more often.
My tendancy is to enjoy and do better at long distances. I like running long by myself, with a group, or in a race, so I end up being more hip than I intend in the environment that emphasizes marathons. However I train, my long distance events always overpredict middle distance performance on a pace calculator, and my middle distance events underpredict my long distance performance. I think I am focused on distances that I do better at. That is mostly just luck, though, because the opportunities exist.