In the section "New Bill Would Change Voting Rights Act," the sentence that shows why pre-clearance is unlikely to be added to the Voting Rights Act is:
"Many Republican lawmakers oppose expanding the Voting Rights Act because they believe it gives too much power to the federal government."
This sentence explains that the opposition from Republican lawmakers is based on their belief that adding pre-clearance would grant too much authority to the federal government. This indicates a key reason why pre-clearance is unlikely to be included in the Voting Rights Act.