Why has Allah described the believers as having come out from darkness, and the disbelievers as having come out from light?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The protection of Allah for the believers from falling into misguidance is considered an extraction from the darkness of disbelief, while the adornment by the companions of the disbelievers of falsehood that leads them away from guidance is considered an extraction from the light of guidance. It may be said about someone who refrains from something that he has 'left it,' even if he was never involved in it. As Allah relates of Joseph: 'I have left the religion of a people who do not believe in Allah' [12:37] (and see explanation of 2:210).

1. The faith of the People of the Book in the Prophet before his appearance was a light for them, and their disbelief in him after his appearance is a departure into darkness.
2. When the miracles of the Messenger of Allah (ﷺ) became apparent, whoever opposed him was exiting from a light he had known, while whoever supported him was exiting from the darkness of ignorance into the light of knowledge.