Final answer:
The Founding Fathers kept slavery in the Constitution due to the Three-Fifths Compromise and the fugitive slave clause, both of which protected and perpetuated the institution of slavery.
Step-by-step explanation:
The Founding Fathers kept slavery in the Constitution for several reasons. One piece of evidence is the Three-Fifths Compromise, which counted enslaved people as three-fifths of a person for the purpose of determining a state's population for representation in Congress. This compromise can be found in Article I, Section 2 of the Constitution.
Another piece of evidence is the protection of slavery in the Constitution. Article IV of the Constitution, specifically the fugitive slave clause, required states to return escaped enslaved people to their owners. This clause prevented enslaved individuals from gaining their freedom by fleeing to states where slavery had been abolished.