It all comes down to quarterback play, mostly. Wide receiver is a position dependent on other players being good in order to get the ball -- quarterbacks being able to throw, the line giving the play time to develop, a running game taking a little pressure off the passing game. It's why you hear the phrase "build inside out" as opposed to starting with the wide receiver position and building outside in.
Prior to this season, the Cardinals have had horrid quarterback play from Leinart and Warner was only OK. It's pretty remarkable Fitzgerald and Boldin have been as effective as they have, and is only a testament to their abilities. Finally, they have a quarterback playing well, and other parts of the team have also improved, and you're seeing them beat teams.
The Lions philosophy might have worked of drafting all those receivers, but it all went to hell in a hand basket once it was proven Harrington couldn't play. They were trying to give their quarterback tons of weapons, but it turned out the QB couldn't utilize them. The fact several other picks were flat busts -- Kevin Jones, Charles Rogers, Mike Williams -- didn't help. But the key was Harrington. Had he been a Pro Bowl QB, the outlook of that team would have changed.
