I believe the answer to "why?" is because back in the day everyone used to spend their first two moves of the game moving the same Pawn twice. In order to speed up the game, they let Pawns jump two squares in their first move. However, this lets Pawns escape having to be captured by enemy Pawns which are on the enemy's fifth rank. As this was not the purpose of the two space movement rule, the en passant rule was added.