The X-Men, Spiderman, Iron Man, and the Avengers are all Marvel characters. Yet you’ll never see Wolverine make a cameo in an Avengers film because Marvel sold off the movie rights for the X-Men characters to Sony, another studio. Even though this means we won’t see some awesome cross-overs Joshua Yehl argues that this is a good thing for comic book fans:

• Only so many comic book movies can be made every year by a single studio.
• Marvel’s most popular characters before superhero movies became a craze were the X-Men and Spiderman.
• If they had held the rights to those characters then they probably would’ve exclusively made movies focused on them.
• They sold the rights before they realized they would get into the superhero movie making business. Once they did they had to scratch around for lesser known characters with potential such as Iron Man.
• And if it weren’t for Iron Man we probably wouldn’t have gotten the rest of the Avengers heroes. Characters such as Thor and Black Widow were unknowns before Marvel began to experiment with movies around them.
• Therefore we have the fragmented ownership of the characters to thank for both the quality and quantity of movies like The Avengers

Read more of his argument over here.

Source: IGN