Because it works. If you talk to any Obama drone they will use the same excuse. Everything is Bushes fault even though Obama hasn't gotten rid of any of the old Bush policies, Obama has just extended and expanded all of it and spent trillions more.
The policies are working. Every sector of the economy is better than it was in 2008 except the GNP but copared to Europe we are in great shape. He is reminding the Republicans who keep blaming Mr. Obama for the current economic problems, that it was another administration that caused them.