This may not be a topic for this forum specifically, but the two examples off the top of my head are in films/tv films so here goes. Basically in tv-film adaptation of Jason and the Argonauts there was this female who was an archer and far more interesting as a character than the conventional princess who had the personality of cardboard and who Jason? was made pre-destined to fall in love with even though the chemistry was more evident with the female archer. Now last time I saw this was in 2001 so I may be remembering things incorrectly but that was the overall impression I got.
Secondly Eoweyn? shield maiden of Rohan. I remember reading the LOTR books and thinking she was a really cool character but for frame of reference I'll just focus on the Two Towers which is a faithful enough adaptation. She was x1000000000 better as a character than the elven bird (Arwyn?) that Aragorn is again almost pre-destined by the logic of the plot to marry. Yet Eowyn gets brushed aside.
Even in Tron, Tron is the dull guy, Jeff Bridges should have got the girl in the computer world.
I notice this time and time again in films/tv where the unconventional secondary love interest is unsuccessful in their aims and the conventional/boring counterpart gets the girl/dude/whatever. What is the reason for this.