The "average" view of the Framers of the Constitution at Philadelphia — many of whom were slaveowners — was that slavery was wrong; that it should be, and one day, would be abolished; but that the task would be difficult, and probably wouldn't happen for a long time. (A "long time" here means, "after they were no longer around to benefit from it.") There was an idea that slavery was slowly winding down on its own (at that particular moment, it was somewhat true), and hopefully one day, slavery would dwindle to the point that it could be ended. All this was rather abstract and distant. It's like the famous prayer: "Lord, grant me chastity... but not just yet."

All this was far removed from the immediate political task, which was to create a stronger national government. Remember, under the Articles of Confederation, the government couldn't even tax people! The idea of a national ban on slavery was unthinkable. If slavery was ever to be abolished, it would surely be done state by state (Massachusetts had done this recently).

The delegates in Philadelphia never came close to abolishing slavery, but they did debate specific constitutional provisions concerning slavery: the Three-Fifths Clause, the Fugitive Slave Clause, and the continuation of the slave trade. All of these provisions are relatively pro-slavery. So, why was that?

The structure of the Convention was that the Virginians and Pennsylvanians were the ones who were really driving the process, with significant help from Massachusetts. Now, how do you get the other states on board? In very broad terms, there are two groups of states to win over: small northern states (Delaware, New Jersey, Connecticut, New Hampshire) — you get them by creating a Senate and Electoral College that massively overrepresent small state interests. Then, the lower South (South Carolina and Georgia) — you win them by throwing in enough pro-slavery sweeteners. That's the strategy.

The South Carolina delegation was organized and effective at demanding pro-slavery concessions. They worked as a well-oiled machine, carefully walking the line between being team players while dropping hints about their willingness to walk out if necessary. (You can actually find writings from the South Carolinians where they themselves are mildly surprised at how well they did.) The Northern delegates probably could have weakened protections for slavery if they pushed harder on it, but instead they spent their bargaining power on other things they valued more. (There were lots of cross-issue bargains at the Convention). Again, this was undoubtedly aided by many delegates' impression that slavery was gradually dying out anyway — or at least, most delegates from Virginia northward were willing to believe it if it made them feel better.