It's very simple. Philly didn't continue to do what it did to have success in the first quarter.
Detroit's play had a lot to do with at, as their bench had no problem hitting outside shots, while philly's bench was unable to provide any offensive help at all.
Philly depends on a transition game, forceing turnovers and stopping second chance shots. Once Detroit started operating successfully in it's half-court sets, Philly (Iverson) lost patience in breaking down Detroits defense. Once your up byt 16 points all you have to do to win is trade baskets, but this Sixers stopped scoring.
The Sixers can play in a half-court game with Detroit, only if they execute properly- meaning Iverson penetrating and dishing, or letting other players handle the ball. AI tries to do too much by himself, and didn't pass out of double-teams quickly enough. The other thing they have to do is defend Detroit adequately. Detroit shot uncharacteristically well, and it is unlikely they will be able to continue to. McDyess shouldn't be able to come off the bench and hit 4 out of 5 shots in a row.
Dalembert played great, but he was all alone out there. Webber and Jackson were a step slow getting over on every play. No one kept Prince from driving the lane. Hamilton was kept under control, but his scoring wasn't needed. AI and Webber scored plenty, but didn't get enough help from the bench, Korver and Igoudala to overcome Detroit's scoring.
It all comes down to Philly executing in the half-court when they have to. They cannot rush shots, or rely on Iverson. They should run a two man game with Igoudala and Webber, or Korver and Webber to let Iverson run off screens without the ball. They should put Dalembert at the top of the key and run the offense through him. Provided the Sixers keep the lane open, and prevent the Detroit defenders from clogging it, they should be able to create plenty of opportunities. As it was, Saturday, Korver didn't even get the ball passed to him- when Detroit is double-teaming and clogging the lane, those shots are open, and Philly has to make Detroit pay, if they're going to win.
They should also press Detroit to prevent them from getting into their offense quickly on the other end. Philly has to increase the pressure on Detoit in their half-court sets, forceing them to pass the ball more and take more time before getting a decent shot.
Unless the Sixers box out and prevent Detroit from getting second chance shots, they won't get any easy transition baskets. No running game, and they're chances are greatly diminished. The pressure is on philly's defense and half-court offence to perform better. They can do it, but Detroit clearly impossed it's will over the weekend.