Why can't a piece (that isn't a pawn) capture en passant? When a pawn makes a two-square move, the en passant rule says that on the next move, any pawn that could have captured it on the single-advance square may do so.

So why doesn't the rule allow a non-pawn piece that could have captured it on the single-advance square to do the same? For instance, in the position

[fen "7k/8/8/8/2n5/8/P7/7K w - - 0 1"]

1.a4

why isn't the knight allowed to respond 1...Nxa3?