Why couldn't congress ban the slave trade but they could tax imports and exports.?

Congress has the complete and plenary power to tax whatever is taxable and the power to tax is the power to destroy. It is arguable that Congress could have taxed slavery into oblivion and perhaps civil war could have been avoided. Look at the heavy and progressive taxation on cigarettes and how that has encouraged many a smoker to simply quit smoking. Of course, sadly, I am a smoker and find noting simple about quitting smoking, so unfortunately pay these heavy taxes just to sustain an addiction. So, perhaps taxing slavery into oblivion would not have worked. The real point is that Congress couldn't ban slavery in the Constitution until Congress existed and at the time the Constitution was being forged there wasn't yet a Congress, (not under the Federalist system created), to ban slavery. Once congress could ban the slave trade, Congress eventually did with the Thirteenth Amendment.