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

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

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)