What happened? A little thing called The Ultimate Universe. Marvel retold their characters' stories from scratch: no decades of continuity weighing them down like Iron Man armor on Captain America.
And therefore no need to admit this happened.
The new version of The Avengers was Mark Millar and Bryan Hitch's The Ultimates. And it kicked your ass in your ass's balls. S.H.I.E.L.D. was now in charge of The Avengers. Hulk was a terrifying threat. Captain America could rout a battalion single-handedly. Hawkeye went from the most laughable member to the only one you could rely on. It was the blueprint for everything that makes the movies so damn good.
Millar and Hitch fantasy-cast their book with some top stars of the day. Like this Brad Pitt-ish Captain America ...
Why, he looks downright pitt-uitary!