because drilling the hole won't make threads on the wood, and if you pay attention to the threads on both screws, you'll notices that the machine screw hasn't the threads as deep as the sheet metal screw, so it won't stay in place for too long, you can go with the machine screw if you use the control horn backplate, but in this case, that is not possible, as you can see, the aileron has some kind of triangle shape, so if you sit the control horn even to the aileron, the back plate won't sit flat and even, so you'll have a weak configuration. that's WHY you should use the sheet metal screws. the control horn backplate may be used in any other flat control surface like the rudder, but not with aileron and flaps.