Because The Avengers had a successful movie. and Marvel wanted to compete with DC's New 52.
Just bad. Huge missing chunks of story all over the place. From the beginning we're to believe that Tony Stark has a scanner that detects the Phoenix Force yet has never gone off despite all the times the Phoenix Force has been on Earth since the Dark Phoenix Saga including an extended stay inside a host (Rachel Summers). They immediately have cause for concern yet there's nothing really telling them just how dangerous it could be. There's no real proof that they know that the Phoenix was destroying planets on the way to Earth and there's no real story purpose for it. It was headed to Earth to fix the imbalance of mutants? Sure. So why did it have to barrel through a dozen planets to do so? Stupid pointless thing to create a false sense of emergency and played no part in the actual story.
The Avengers, instead of asking the X-Men their advice on how to proceed (including failing to consult at least 2 - 3 former Phoenix hosts) listen to Wolverine who just recently had a major falling out with Cyclops yet take his words as gospel and demand they hand over Hope despite not really having any plan on what to do with her. Then...FIGHTS!!!!
Fighting stresses out Hope and when she finally gets the Phoenix can't handle it. Tony Stark,not understanding the Phoenix, just rams a giant Iron Man suit into it and creates the Phoenix Five who seem benevolent and wants to "fix" the world. Of course that doesn't include fixing mutants because the writers forgot about that plot point so it's never brought up in the story. Ever. Avengers feel emasculated and keep attacking the Phoenix Five to the point where they start driving some of them nuts. Also they "capture" one of Hope's lights which was stated was necessary for her to control the Phoenix properly. Only they completely misplaced her and she's never seen again through the course of the event. At best, she drowned when Namor attacked Wakanda.
In the end Iron Man gives up and claims that they just need faith which was basically the X-Men plan but since it's Tony Stark now that plan is apparently genius. So Hope gets the Phoenix, everything is great and everyone wins...except the X-Men who are either on the run, locked up or all but officially indoctrinated into the Avengers.