Why did the disputes over authority after Muhammad's death hinder future Muslim unity?

I do not know what Chapter 6 of your book says, but I
would say that the answer to this is that the disputes over authority after the death of
the Prophet Muhammad led to the split between Sunni and Shia
Muslims.

After the death of Muhammad, one group of his
followers felt that there should be an election to designate his successor.  This group
chose Abu Bakr and they would later become the
Sunni.

Another group of Muhammad's followers believed that
only God could pick a successor to Muhammad.  They believed that Ali, Muhammad's
son-in-law and cousin, was chosen by God.  This group became the
Shia.