I think a large part of it boils down to how marksmen as a class tend to circumvent design rules that have been applied practically everywhere else. In a League where a significant amount of champions, including entire subclasses, have received major changes to make their damage less reliable, marksmen remain a class capable of dealing massive, constant, unavoidable damage at no resource cost, for example, and don't really seem to pay for it with reduced reliability elsewhere. Diversity is held as an important design tenet, and Riot has deployed significant efforts towards making matches more diverse, including controlled randomness in the form of Drakes and plants, yet the eternal marksman-support mandatory duo bot lane plays a large part in limiting match diversity overall, as it not only makes for a pretty unchanging lane setup, compared to the class diversity of top, mid and jungle, but also tends to warp entire team comps and even the entire metagame around accommodating marksmen. The marksman class is supposed to have their early game as a core weakness, yet a great deal many marksmen are viable in solo positions, to the point of even becoming lane bullies against champions with less range and mobility.
In the current state of the game, marksmen aren't as strong as they used to be for a large part of Preseason 6, but the issue isn't so much with their power, but that they could be even weaker and still be considered mandatory every game. Unless marksman damage drops off so severely as to be weaker than that of other classes, which will never realistically happen with their itemization, marksmen will still be the best at taking towers and objectives, and will still be the best class for teamfight DPS, which means they will never cease to be present and central to the game, while other classes often fight for representation, let alone a chance to take the spotlight.
There are a lot of issues tied to this that makes solving this kind of problem difficult, and a lot of them are structural: the game is still a state where, eventually, the only major way to influence the map becomes high DPS, especially ranged DPS when you factor in towers. This isn't just a problem with marksmen, and it causes tanks and supports, the only two classes not tied to damage, to end up feeling disconnected from the environment past a certain point. Marksmen also likely have too much generic power and reliability, which causes them to fulfill multiple functions at once (objective control, teamfight DPS, etc.), functions that would usually define individual subclasses, and the class still needs to be split into those, which should reduce overlap.
