According to Kant in the Foundations of the Metaphysics of Morals, why are moderation, self-control, and sober reflection not unconditionally good? Can you give me an example as well?
According to Kant, all of the things that you mention can be good, but they can be bad as well. These are all personal attributes that can be used by people to do very good things. The problem is that they can also be used by bad people to do very bad things. Since they can be used for evil, they cannot be unconditionally good. The only thing that can be unconditionally good is a good will.
In order to think about how this is true, think about examples. Of course, a person can act in moderation and with self-control and deliberation for the good. We could argue that President Truman acted in this way after World War II. We could argue that, instead of punishing Japan and Germany harshly, Truman and other American leaders controlled their desire for revenge and instead helped those countries rebuild. This helped the world as a whole.
But not everyone who acts with self-control and cool deliberation does so with good intent. Evil people can also act in these ways. Kant says, speaking of the qualities you mention,
… without the principles of a good will, they may become extremely bad, and the coolness of a villain not only makes him far more dangerous, but also directly makes him more abominable in our eyes than he would have been without it.
Think about the Nazis. They did not simply act impulsively, walking down streets and abusing Jews that they happened to come across. At the highest levels, they acted with cool deliberation, planning out how to most efficiently carry out the Holocaust. Their “coolness” made them “far more dangerous” to the Jews than they would have been if they had simply been thugs carrying out random violence. It also makes them “abominable” to us because they committed their acts of evil even after they thought carefully and cooly about them. This is why we think that “crimes of passion” are less evil than things like premeditated murder.
To Kant, then, these kinds of personal qualities are only good if they are put to good ends. Because they can be used for evil by people like the Nazis, they cannot be unconditionally good.