That is, what is the actual reasson this was written into the rules? I'm not very into chess so pardon me if there's an obvious answer.
There is no obvious answer from what research I have done. It was very likely added simply to make the game more interesting.
The en passant rule was then added to make it harder to use the two space rule to avoid getting captured.
The reason is history.
At some point in history (between 1200 and 1500 AD in Europe when the old game of Shatranj evolved into Modern Chess), there was a lots of experimenting with different rules in chess, and the double move of the pawn made the game more dynamic and pleasant for the Chess players, therefore it stayed.