Sam blamed a lack of resources. "Stark's not here, none of us are engineers. Where would be get building supplies? And there are unions for this kind of--". In short, the problem was Tony's absence.
As for Miles and the other Sam, Nova says "I'm done being ordered to punch people I like, and so is Spidey!" - sounds like Civil War II was directly to blame for their exit.
And I'm not surprised people didn't know about this. ANAD Avengers didn't have a proper ending because the last three issues were single character one-shots. They really should've shown the team's break-up in a final issue of ANAD Avengers. Instead, they split the story across Champions #1 and the new Avengers #1 (bear in mind, ANADA, Champions and the replacement Avengers book were all by the same writer, Mark Waid).