Let's not forget the most recent example of Scuderia non competitiveness, the F2005. What does the performance failure of 2005 have in common with the failure of 2009, not necessarily on a mechanical or technical note but on a management level?
2005, Ferrari failed massively because mainly due to the 1 tyre ruling, and also some changes to the diffuser ruling.
+1 In a thread I started in the F1 section a while ago I mentioned a brilliant article that talks about that. The superdiffuser works in combination with the lower wing of the rear wing: It channels the airflow in a way that it creates downforce. So the air flow between rear wing and superdiffuser has to be optimized, which it is not for the Ferrari (and Renault) because of the way their gearbox and exhausts are set up.