And this reason is why Mary Sue stories makes poor writing.
Most of the time, Mary Sue writers get too intent to pile awesomeness upon awesomeness for their Sues to "shine" more that the narrative suffers. Instead of an engaging story, we get is a list of cool things that the character has done. And considering how anything can happen in the fictional world, this list of amazing feats fail to impress.
The presence of Mary Sues are especially irritating in fanfics because they frequently warp the universe of an existing fandom to an unrecognizable form. In other words, they invade the fandom.
People who read fanfics come in with certain hopes/expectations of the settings, rules, and characters - but in Mary Sue stories, all of this would have been changed. Instead of seeing our favorite characters in action, we see them abandon their personality and goals that made us like them in favor of an Original Characters we don't know, let alone emphasize with. We came for entertainment, but we are forced to see someone else's shopping list instead.