173 ( +1 | -1 ) Because...It might help to understand EP by considering why the move was introduced. Imagine a black pawn on d7 and a white pawn on e5.... early chess rules only allowed a pawn to move one square at a time but later, this changed to 2 squares on the first move. So, why EP?
In the old rules, the black player moves his pawn to d6 and of course with black pawn on d6 and white pawn on e5 white has the option to capture the pawn by e5xd6 or simply play another move and elect not to capture.
However, in introducing the new rule to advance a pawn two squares and immediately play d5 it was recognised that white would be disadvantaged because the option to capture the pawn that existed when d6 was played has now gone. EP therefore allows white to invoke his rights under the old rules while accommodating the new rules to advance a pawn two squares. Thus when black plays d5 he is (implicitly) sayings to white if you like, image my pawn has stopped on d6 do you want to capture it? While you think about that, I will move the pawn to d5. This is where white can now invoke EP and capture the pawn on d5 (as if it were on d6.
Note in passing, a move sequence of e5, d6, some other white move followed by d5 produces the same pawn configuration as we have just looked at before but here the rule of EP cannot be invoked since white has already been offered to option to capture the black pawn while it was on d6 and this option was declined when white played some other move. So, EP must be applied at the time the pawn advances the two squares and the opponent had/has the option to enforce the logic that it has only moved one square and I am going to capture it..
Hope this helps. Apologies if I have bored anyone :-0