Why is en passant only possible in the next move?

Lol it's as if people assume there was just a pressing need for a rule named "en passant" and picked a move that best fit the term. 

This is the correct answer. Saying that the rule is the way it is because of its name is nonsense though.

ignoring the fact that the name does mean "in passing", lets look at it from a purely logistic point of view. if you were able to capture a pawn pushed 2 squares up at any point after that move, it would lead to many arguments in later game over whether that pawn was pushed up one square at a time or not. "so lets simplify it," says some guy. "let's just make it so that you can always capture a pawn with another pawn by moving to the square behind it". at this point, making a pawn push becomes almost impossible, the game becomes overly complicated and the pawns become unbalenced, and pawn structure becomes almost unworkable.

long story short, the way en passant works was done so that the pawns stayed balenced and the game stayed workable.

And finally, it would be fantastically hard to keep track of. Is the black pawn on e5 there because it first moved to e6 and then e5? Or is it there because it moved directly from e7 to e5? If the former, then you can never use the en passant rule to capture it. If the latter, why should you be allowed to make all kinds of other captures and developing moves and then decide six moves later that you'd like that pawn after all?

Plus, if I'm moving my pawn out two squares, I KNOW the en passant rule. I know that my opponent can take it, but my strategy tells me that RIGHT NOW it would be a mistake for him to do so. I can't be expected to see many moves into the future where, at some unknown point, it suddenly becomes the right move for him and thereby loses the game for me. It's now or nothing.

In the beginning, but once a set of rules is established, rule changes are no longer arbitrary. 

I begin to believe that names work magically.

It's amazing how many people think the name of the rule is an adequate explanation.

as i said, forget the fact that it is called En Passant. being able to take a pawn that has advanced by 2 at any point after it does so would break the game. no one would ever move up by 2 as it would leaving a hulking weakness in their pawn structure every time they did. we would essentially go back to the point where pawns could only ever move by 1 square at a time. and that's not even mentioning the arguments that people would get into over whether that pawn they just capture went straight to e5 or if they in fact played e6 then e5. (yes you could check the record, but that would take time, and in a tournament, an arbitrator would have to be involved.) so, it is just simpler and more balenced to do it the way it is in the rules now.

It's the capturing Pawn that is "passing". When the capturing player takes a Pawn "en passant"... the move the captured Pawn had made would be... in the past, ie not happening now.

I got a few counter-questions for the morons that think you should be able to en passant any time you want to:

1) Does my pawn have to be on the 5th when Black moves his pawn?  Or could I play 1.d4 e5 2.d5 c5 3.Nc3 d6 4.dxe6, or would only 4.dxc6 follow your rules?

2) If Black moves his pawn again, can White still do en passant?  For example, let's say White's pawn is on g5, the only thing on the h-file is Black's h7-pawn.  After 29...h5, given your rules, if I don't have to play 30.gxh6 in order to en passant, is it available for ever, or only as long as the pawn is still on h5?  What about after some other move by White, maybe 30.Kb1, and Black plays 30...h4, 31...h3, 32...h2, can I now play 33.gxh6 and remove the h2-pawn?

These two questions should explain why your hypothosis of being able to do it any future move is moronic!

Okay but the entire point of this thread was for the OP to understand why the rule is the way it is.

Beyond what's already been mentioned, it would get incredibly confusing otherwise. You'd have to constantly be keeping track of which pawns had double jumped to the 4th/6th file, and which ones had done it in two moves.

everyone keeps repeating what's already been said, so i'm UNTRACKING. bye!

It was done to passify the pawns. They were quite upset that they were the first to be sacrificed with this barely obtainable reward of being promoted. 

SO they were given this ability so they could feel more like the big pieces.

OK, why this ability is limited to only capturing pawns while in passing, then I just don't know.  If a rook or a queen traverses a capture square of one of my pawns why can't I catpure it similarly?

A well thought out treatise.

So is resigning. I think players should not be able to resign. They should fight to the death.

I'm not big on the way the knight moves.  I don't have a good alternate suggestion, but we should definitely abolish the current way.

The knight moves in an 'L' shape. I would suggest changing it to another letter like 'Z' or 'O'.

Nobody can do it all. Wolverine can't fly like Superman but it doesn't make him any less a super hero.