@BeforeTheFunk you are right, I was too hasty with my comment.
@averagepleb2 Sure white can play 2 a4, but that move contributes nothing to development. 2 Bc4 at least develops the Bishop. Moreover, since 1...c5 hinders 2 d4, it seems logical to settle for d3 instead and then it makes sense to put the bishop outside the pawn chain first. Granted 2 Bc4 is not popular any more, but I would not be surprised if some top player picks it up again.
@averagepleb2 Sure white can play 2 a4, but that move contributes nothing to development. 2 Bc4 at least develops the Bishop. Moreover, since 1...c5 hinders 2 d4, it seems logical to settle for d3 instead and then it makes sense to put the bishop outside the pawn chain first. Granted 2 Bc4 is not popular any more, but I would not be surprised if some top player picks it up again.