Why hasnt a decent X-Men: Origins movie ever been made?

[Meehl]

The X-men were interesting,but not because they fit the mold of the superhero genre. They never fit the super hero mold. That's the Avenger's niche, which helps in movies but has made their comic awful since issue 1. The X-men were interesting starting with ANAD X-men because they were interesting characters who were further developed in the background of sci-fi/superhero stuff. First class stayed true to what made the X-men interesting: Interesting people in conflict who happen to have super powers.

In other words, the X-men would be interesting even without super powers because of the character dynamics. The Avengers are vacuous even with their shiny super powers.

[Drax1981]

This is gonna be my answer to everything now but they need Joss Whedon to write an X-men origins movie, from what ive read he's a big X fan and he had that run on Astonishing x-men, i think he'd be perfect.

He would make a good choice but the issue I'd have is he came into a movie with a lot of the heavy character work already done. Cap, Thor and Iron Man all have their own movies in which to grow as characters. Hulk grew into three separate characters. All the characters got spotlight but none of them really hard arcs.
First Class ran into this problem too. Mystique had the closest thing to an arc but Havok? Darwin? Why did Angel make the choices she did? Why did the team of inclusion feature all white people and self-loathing Beast? There's only so much time and priorities have to be made.
Anyway, I'd say if Fox was serious about the X-Men universe they should follow the Marvel model and get singular movies for the character work and leave the big action stuff for X-Men proper.