Why did God not protect the imams and scholars from error in matters of religion? And why did the Prophet, peace be upon him, not explain everything during his time?

The religion of Allah is preserved, and differences in subsidiary jurisprudential issues do not mean its distortion. Rather, it is an expansion for the Ummah and an opening for the door of ijtihad (independent reasoning). Allah Almighty has made some rulings presumptive to provide ease for those obligated.