Instead of trying to isolate white king into a corner for checkmate?
Never heard of a rule like that. Are you sure?
king versus king; king and bishop versus king; king and knight versus king; king and bishop versus king and bishop with the bishops on the same colour. - wikipedia
EDIT: There would be positions where you don't try position the king to try to hold the opposing king in a corner where the opposing king can't move towards the knight to escape a check, but in those positions it will be able to move away in other directions.
It's not in the written rules of the game, but is a consequence of the rules.
To place the opposing king in checkmate, you must have sufficient pieces to threaten the square the king is on, and all adjacent squares, simultaneously. This means anywhere from 4-8 squares depending on the position.
King and knight, alone, cannot threaten enough contiguous squares simultaneously to cause checkmate in any position, largely because the knight must place the opposing king in check (a king cannot get close enough), but due to the mechanics of the knight's move there will always be at least three adjacent squares not threatened by the knight at that moment, and at least one that cannot be threatened by the knight's king.
You should feel free to try this yourself: place a king of one color on the board, and a king and knight of the other color, and then use only those pieces to try to construct a checkmate. It simply cannot be done. The best you can do is isolate the lone king in a corner and cause a draw by depriving it of moves (but it will not be in check at that time, so it will be stalemate, not checkmate).
And as luiz-pv9 points out, king+knight is not the only "insufficient material" combination.