Why, because the king isnt even in check ? If the king is not in check, obviously it cannot be checkmate.
Also, its not legal to move the king in check. Thats why you deprived the opponent king of any legal move. There is also no other piece the opponent can still move.
So you have caused your opponent to not have any legal move, thus its stalemate and a draw.
Its very common for beginners to draw a completely won position this way. They have decisive advantage but for some reason refuse to actually go after the oponents king.
I’m white btw