Answer

In general, you could ask this about the whole matter of repentance. Why does it merit a separate section and not get included in the laws of the foundations of the Torah?
I explained in the past that the laws of repentance include everything that is not part of formal Jewish law but still must be done (aside from character refinement, for which the laws of dispositions are devoted). In the laws of the foundations of the Torah (after the philosophical introductions), only actual laws are included.
From this, one can understand that to repent means to return to being someone who chooses. That is the essence of repentance. That is why he included free choice as well in the laws of repentance.