X-men and fox
 

Why Marvel sold the rights to fox ?

If I remember correctly, Marvel wasn't doing too hot financially in the late 90s so they sold the rights to some of their more popular characters (i.e. x-men, spiderman, punisher, fantastic four, etc) for added revenue.

I doubt they will bring back the rights sooner.

I think Fox has made the series better with The Wolverine, First Class and Days of Future Past. I think the X-Men can hold their own universe without other characters. Also I remember Fox got the movie rights to X-Men in the early 90's thanks to the TV series being popular on Fox Kids.

What about fantastic four ?

Well Fox is rebooting the Fantastic Four, I guess if it bombs they may just let Disney have it.