Whedon is (or now perhaps was) a middle-level player in Hollywood,
Now this is idiotic. Even I've heard of his work and I almost never see movies. "Buffy the Vampire Slayer", "Firefly", "Toy Story", Wikipedia lists even more stuff going back to "Roseanne". His work on "Speed", "Waterworld", "X-Men" all predate "Iron Man" as the catalyst you insist made him get the job.
And that's not counting his work in comics, the medium that's been such favorites of moviemakers for years now. But apparently he told Marvel/Disney/Hollywood he was going to direct "The Avengers" and, being the middle-level player you insist he was, they handed over the keystone of the expensive franchise without hesitation.