Amir is better suited to the starting 5, cleaning up DD/Lowry/Ross misses, and generally just setting the tone for the starting lineup who has shown a tendency to become lackadaisical without Amir firing at a full clip. This has been especially evident recently.
PP is the better offensive player. If Amir goes to the bench he is immediately given more offensive responsibility which wouldn't facilitate his success. The guy lives off of putbacks and he's a great finisher around the rim so it just makes sense for him to be in the starting lineup, and having PP get hot off the bench.