Black doesn't develop the light-square bishop because White can then play 3. c3, which develops White's queenside and also extremely advantageous forWhite. After 3. c3, Black would have to waste a tempo with ...e6; meanwhile, White is already castling kingside and has a strong pawn center. The other reason Black doesn't develop the light-square bishop is that it's not really well placed on e7 - it doesn't have any good squares to go to. For example, after ...Nbd7 followed by Rd8 or Qc8, White will just take back with the knight and get an even better...