Socrates claims that he has always had within him a divine voice which sometimes stops him from saying or doing something, but never urges him to do anything. why does he think the voice has usually stopped him from participating directly in the political life?

This "divine voice" seems to be a manifestation of his own conscience/moral compass. Not entering into political life was likely a deliberate choice as that would hold all of his actions accountable to the well-being of his community, and thus, his mistakes would be punished in a different way than if he remained a private citizen.