Maybe because they CHOSE not prohibit secession? As I have already said several times, if they wanted a prohibition on secession in the document they would have PUT a prohibition against secession in the document.
You claim they discussed the issue of secession at the Convention or elsewhere I would like to see where you have seen any such discussion. I have read Madison's notes and Hamilton and see NO reference to that topic.
In fact the ONLY reference (though not called secession) is in Washington's Farewell Address which warns about listening to those who would advocate such a thing.