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A more natural question would be: "is there any reason why checkmating with a pawn drop is forbidden in Shogi?". You can checkmate by dropping any other piece, so why would a special exception for Pawns be needed? Or for drop moves, as it is perfectly allowed to checkmate with a Pawn move on the board. It is a highly irregular rule exception. Unlike in Crazyhouse, where all pieces are treated the same in this respect. (But there it can be considered strange that Pawn dropping on the back rank is not allowed...)

That Shogi forbids dropping of Pawns in files that already contain one is understandable from the fact that Pawns capture straight forward there: without this rul you would quickly build an impenetrable fortress of doubled Pawns around your King, and the game would become unplayable. I never saw any detrimental effects of allowing Pawn-drop mates, though.