Why wasn't Hatake Sakumo (Kakashi's father) revived?
It wasn't mentioned anywhere. Kabuto never talked about Sakumo when showing off or discussing his reanimations. 

We can only assume:


They never found a body, or it was so well guarded that Kabuto couldn't get a DNA sample.
Kabuto was sure of his victory even without Sakumo's DNA, or waited for a right moment to pull him out as a trump card.
Kabuto is an idiot. (He is, what the hell did he think was going to happen when he revived Madara?!).