Both 1...e5 and 1...e6 2.d4 d5 accomplish the following three things:
- occupy the center with a Pawn
- open diagonals for the Queen and the King's Bishop, help to develop these pieces
- get Black one move closer to castling
But the difference is that 1...e6 2.d4 d5 accomplishes these three things in two tempos instead of one, so it makes Black lose one tempo. Why would you do something in two tempos when you can do it in one?
Other differences between 1...e5 and 1...e6:
- 1...e5 prevents White from playing d4
- 1...e5 allows Black to develop his King's Knight to it's optimal square, the f6 square, without fear that White will kick it away by advancing his e4-Pawn to e5
- 1...e6 blocks the Queen's Bishop, turning it into a bad Bishop
1...e5 is better than 1...e6 in every single way.
1...c5
- does not occupy the center with a Pawn
- does not help develop any piece (except the Queen, but there's an opening principle that says "don't bring your Queen out too early")
- does not get Black any closer to castling
- occupy the center with a Pawn
- open diagonals for the Queen and the King's Bishop, help to develop these pieces
- get Black one move closer to castling
But the difference is that 1...e6 2.d4 d5 accomplishes these three things in two tempos instead of one, so it makes Black lose one tempo. Why would you do something in two tempos when you can do it in one?
Other differences between 1...e5 and 1...e6:
- 1...e5 prevents White from playing d4
- 1...e5 allows Black to develop his King's Knight to it's optimal square, the f6 square, without fear that White will kick it away by advancing his e4-Pawn to e5
- 1...e6 blocks the Queen's Bishop, turning it into a bad Bishop
1...e5 is better than 1...e6 in every single way.
1...c5
- does not occupy the center with a Pawn
- does not help develop any piece (except the Queen, but there's an opening principle that says "don't bring your Queen out too early")
- does not get Black any closer to castling