Because what your employees do is ultimately the boss' responsibility in cases like this. He said that at the time, back in June. He said, "Although I was not aware of the contents of the memo prior to its distribution, I consider the entire campaign -- and in particular myself -- responsible for the mistake," in June.
http://findarticles.com/p/articles/mi_qn4155/is_20070619/ai_n19305385What else do you want him to say? It looks like Obama is better at taking responsibility for his actions and admitting mistakes than Clinton is.