ComicBook 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.
ComicBook Bottom line: Who won? (spoiler warning)
When we first saw the teasers for Avengers vs. X-men, it was stated by the Marvel staff that unlike the previous giant story arcs (like the Civil War), there would actually be a winning side. With the final issue of the main story released, did Marvel actually deliver on their promise?
Primary objectives
Avengers: The Phoenix is coming, we must protect the Earth at all costs.
- This has been the primary mission of the Avengers since the beginning and they never once strayed from this goal. Of course, at the end of the series the Earth has been saved and most of the damage has been amended. Despite what you think about there efforts, did the Avengers ultimately win?
X-men: The phoenix is coming, in order to restore the Mutant race (and prevent Cable's bad future) it must be harnessed by Hope.
- The Avengers got the Earth saved as they wanted, but it couldn't have been accomplished without Hope harnessing the Phoenix force (the X-men's orginal plan). Not only was this accomplished but Hope manages to restore the mutant race at the same time. Did the X-men technically win?
Brief Analysis
From a superficial perspective it seems like we're supposed to side with the Avengers here. Even from the beginning, the Avengers are like rational minded straight-men while the X-men come off as zealous followers of a cult. The latter discussing their plan from a more lofty or hopeful standpoint; despite the fact that there were some legitimate reasons for using the Phoenix force. Many of post-schism X-men on Wolverine's side sided with scott out of loyalty to him, rather than any practical reasons (except Rachel, who harnessed the Phoenix Force in the past). Eventually, because of iron man's actions, 5 Phoenix hosts were created. Here their actions can be interpreted as either making the world a better place, or a dictatorship (I'll weigh in on my opinion in the comments). But at that point they did nothing evil until they were provoked by the avengers. Each host was eventually defeated after they performed some heinous act. Eventually Cyclops was all alone, and while he was the one who seemed to have the most control he still eventually followed suit to the other hosts, and was defeated.
It seems that marvel copped-out with an everybody wins ending. It was Scarlet Witch and Hope (mostly) whose actions stopped Cyclops.
ComicBook They Just Don't Care
What can you say when Marvel comes up with a plot and barrels forward with it even when the basic premise is false. Not even in the "this obscure comic from the '70s says this..." but in the sense where a major character starring in a title being published currently renders the entire crossover ridiculously false on its most basic level. Avengers vs X-Men meet Rachel Grey. I have to hand it to the writer of "Wolverine and the X-Men" as he attempted to make sense of a plot that only works if you don't know that there was a Phoenix host active on Earth for decades, before eventually throwing his hands up in the air and surrendering to the stupidity.
Of course that is just the rotten core of the series. Sprinkled on top is the transformation of the Avengers from heroes into out and out super villains. Captain America mobilized an army, invaded a sovereign nation in an effort to use extraordinary rendition on one of its citizens (and was associated with an assassination attempt on that citizen), when he failed in his abduction ploy he engaged in an act of military aggression which resulted in the near total imprisonment in concentration camps of the world's smallest and most vulnerable minority population, and violated their individual autonomy in order to remove their powers. The Avengers have zero moral authority, and everything the X-Men did (from Magik's hell-prisons to Namor's invasion of Wakanda) was justified by the Avengers unprovoked aggression (not to mention war crimes). Thank you Avengers vs X-Men for taking my beloved Captain America and making him a fascist (and that's not hyperbole).
Avengers vs X-Men's main crime is the same as Civil War's, it is unconsidered. The writing team didn't consider the history of their characters. They didn't consider the actions of their characters. They didn't consider the Marvel universe. It's sloppy, and resulted in another terrible event.