Answer:
Because it was the only way to get all 13 states to agree to the Constitution, and the founders wanted all 13.
Step-by-step explanation:
Slavery was fundamental to the US economy in the 1780s. Northerners traded slaves and carried slaves from Africa to the US in their Nortern ships. In the South, agriculture was the main economy, and it all depended on slave labor.
If the US Constitution abolished slavery, there was no chance of getting all 13 states to join. Instead of uniting the 13 states into one government, the country would have been split apart.
Of course, in 1861, the country WAS split apart by the US Civil War. The Civil War came about mainly over the issue of expanding slavery into the Western Territories.