It's because the only apperance of him is a HUGE spoiler to the game... There would be no way they could take a picture of him with out ruining KH2 somewhat. He also doesn't really show up til the end. KH1's book never showed Ansem yet he was a main character.
Eh... To be honest he doesnt play an important role (or show up) till the end of the game. The manual always covers the first like... 2 hours of a game if that. I'm pretty sure it does mention him though.