The difference between Lucina and Dark Pit is the formers changes effects most attacks while the latter effects only two. Between Marth and Lucina, Lucina is more beginner friendly due to her attacks being consistent, however because of that consistency she can't do techniques like Marth can by manipulating the sweet and sourspot spot of the Falchion. To a generic player, they prefer one or the other based on which character they feel more comfortable with. Want sweet and sour sauce shenanigans? Pick Marth. Want a more vanilla approach? Pick Lucina.
It also applies to other clones too. Want to be a jack of all trades? Pick Mario. Want a more mighty glacier/glass cannon take of said idea? Pick Doc. Want power over speed? Pick Link. Want speed over power? Pick Young/Toon Link.
With Pit and Dark Pit, they feel too interchangeable. There are differences sure, but ultimately their playstyles are the same. Attacks hit with the same power, speed, and range, with no variables to it. Same thing with their movement. So the sense of preferences is blurred, which in my opinion is a bad way to make a clone character. Clone characters should be viewed as alternatives, with the choice being that you like the moveset but you may prefer a different playstyle accompanying it.