Because en passant would have been available if your pawn were on c5.
But it is not available since your pawn was on c6.
En passant (French for "in passing") is a special chess rule allowing pawns to capture a pawn that has just passed it.
This is not a bug or a hack but a legal chess move that has been part of the game for over 400 years and an official chess rule since 1880.
Example of en passant
In the example below, Black has just moved his pawn forward two spaces, landing alongside the white pawn. What many beginners don't realize is that White has the option to capture the black pawn as if it had only moved one square instead of two:
Do I have to capture en passant?
White doesn't have to capture in the scenario above, but if he doesn't, the option won't be available next turn.
Imagine it as if the black pawn is moving past the white pawn. In the moment while it is moving past, White has the chance to attack it, but only in that moment. Once the pawn has successfully moved past (the turn is over), it can't be attacked by en passant anymore!
When en passant is played you will see the 
icon next to the move in the moves list:
Hi guys.
I was playing this game and on move 21, my oponnent goes pawn to b5. I was hopping to en passant, but the game didn't let me do it.
Does anybody know why ??
Thanks !