Why didn't the Founding Fathers abolish slavery during the Constitutional Convention?

While the founding fathers didn't abolish slavery, they did start the process by adding a clause saying that the slave trade would become illegal in a few decades. They did this so they didn't have to deal with the problems this issue the problem presented. (They were lazy)

They wanted to obtain votes from states that were pro-slavery and by abolishing it they may have ruined the chances of receiving southern states votes for a declaration of independence