Black doesn't develop the light-square bishop after 1. d4 d5 2. Nf3 because 2...e6 is a more solid and flexible move for black. After 2...e6, black has the option of playing 3...Nf6 (which develops the knight to a good square), or 3...Be7 (which prepares to fianchetto the black king's-bishop). Both moves are good, so black can choose based on what he thinks will be best for his position. 3...Nf6 also has the added advantage of preventing White from playing 4. e4, since that would attack Black's d-pawn (and leave White with a weak pawn on e4