User blog comment:LoveWaffle/Strangely Synchronous Dr. Strange, Jessica Jones, and Deadpool Updates/@comment-24901406-20141210213243

Ryan Reynolds is back because Fox aren't in a position to sack him.

What few people realise is that Deadpool is a separate franchise from the X-Men which Fox had to go out of their way to get, by which point it was already in the hands of another studio (New Line if you are interested) who had already cast Reynolds in the role by the time Fox acquired it. While obviously we don't know the specifics of the deal, we do know part of the agreement was that they were producing "New Line's film" so had to take on Ryan Reynolds and the crew that had already been taken on. Over the years they've all fallen away and left the project... except for one.