Marketplace Business: The Comic Book Battles Behind the Big Screen

Then Marvel decided it wanted a piece of the action, and the company came up with a plan to buy back its characters and to make its own movies. But there was a wrinkle. While Marvel got Iron Man back from Paramount and Sony gave up Ghost Rider, Twentieth Century Fox refused to sell back the rights to the Fantastic Four or the X-Men.

That’s when the arguing, and ultimately the lawsuits, began.
— Sally Herships

A succinct explanation of why Spider-Man coming to the MCU is such a big deal, and why you shouldn't expect to see Wolverine popping up there any time soon.