Nicodemus may have approached Jesus in the night as a sign of respect, vs. debating him openly in public. This may have been a way to treat him as a peer. In any case, this moment was used to teach the Pharisee, and Jesus laid out some pretty complicated theology for him. This should have been right in his wheelhouse, because as Jesus pointed out, Nicodemus was indeed a teacher himself. The message of Jesus was often simple and told in stories, but he rose to the occasion with a more learned mind was before him.