The omission of the word "slave" or slavery" in the Constitution suggests multiple things about the founding fathers.
1) It was an issue they did not want to address at the time- Slavery was an institution during the 18th century that was widespread throughout the United States. This institution did not become synonymous with the Southeastern states in the US until the middle of the 19th century.
2) It was an institution that they thought would eventually become phased out- At this time, the Southern states had been growing tobacco on these lands for over a century. This labor intensive crop had ruined numerous acres of land in the US. Many of the founding fathers thought that this system would no longer be effective and that it would fade out on its own. The cotton gin, created in 1794, is the machine that allows slavery to become extremely profitable again. This machine causes a desire to not only keep slavery but it expand the institution into new territories that the US would acquire.