As a designer you have a choice:
- Have the prop operate in clean air, and the wing behind it in disturbed air, or
- have the wing in clean air, and the prop behind it in disturbed air.
Pusher prop designs are generally that way because the designer thinks that they have a very good, clean, laminar flow wing - and to maximise that efficiency they want the prop behind.
For something like the Piaggio Avanti, it also moves the CofG back, giving a nicer cabin, and getting the prop out of the way of the cabin (easier entry, and quieter operation).