Quicksilver isn't a character, he's a plot device in DOFP, and it's by and large most people's favorite scene in the movie.
They throw in an easter egg to hardcore fans about his relationship to Magneto and his little sister, but the movie never even bothers to explain how they find him, or why he's not helping in the final fight.
Audiences simply do not care about origin stories explained in exhausting detail.
How many people that saw Avengers actually watched Ang Lee Hulk or Incredible Hulk, two underperforming flops? And yet, they got the basic run down, he's a smart guy who turns into a monster when he's angry. Here, watch him CGI smash stuff.
The Joker never had an origin story in TDK. He just shows up one day, and he's the Joker, and he hates Batman. He's a great character, who's obviously a bad guy, so the audience roles with it.