I like to think that FlutterDash is so highlighted, because of all the Mane6, these two have been friends the longest. While Pinkie and Twilight didn't know the other Mane6 until they moved to Ponyville, Rarity and AJ did know each other (both living in Ponyville and going to school together), but weren't FRIENDS until S1E1, and their friendship didn't really take shape until the sleepover at Twilight's episode. But Flutter and Dash? Those two were BFFs at least since Flight Academy.