Why didn't Booker remember *SPOILERS*

Look back at the tagline when the game starts " the mind creates memories where none exist". Booker's mind starts creating false memories from the memories of his previous life, since we play as the 122nd Booker I believe in which he attempt to rescue elizabeth. Where we are seen in the ending that Robert says "bring us the girl and wipe away the debt" is reffering to the baby, when the game starts he creates the memory that he has to bring Elizabeth.

The baptism was before he sold her away I believe.

Well you'd have to be pretty stoned to give a baby to a head of lettuce. I don't think I'd remember either.

Plus, traveling between dimensions alters one memories, creating memories that don't exist. The same thing happened to Robert Lutece when Rosalind pulled him into her dimension. Luckily for Robert, Rosalind was able to heal him.

Robert and Rosalind pulled him through to their dimension and told him to bring the girl to New York. He was never hired and therefore Dewitt changed events in his mind to make sense of things.

The first flashbacks of Dewitt's office show how he remembered the event after his jump. Then later Elizabeth show's him the event as it actually occurred.

The baptism is the point at which Booker becomes Comstock. If Booker is baptized, he becomes Comstock and Elizabeth never exists. If Booker refuses the baptism, Comstock doesn't exist and he goes on to have a daughter, Anna, who ends up being taken to another timeline to become Elizabeth.