insufficient material

Because you could end up in a position like this:

According to FIDE rules, if there is a self-mate, then there is sufficient material.

From wikipedia article on draws in chess, emphasis mine:

• Impossibility of checkmate - if a position arises in which neither player could possibly give checkmate by a series of legal moves, the game is a draw. This is usually because there is insufficient material left, but it is possible in other positions too. Combinations with insufficient material to checkmate are:

• 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 color. (Any number of additional bishops of either color on the same color of square due to underpromotion do not affect the situation.)

sad that you can't force king + 2knights vs. king on mate