So why was it still allowed?
No ability to enforce.
The Articles were just too weak.
https://constitutioncenter.org/blog/10-reasons-why-americas-first-constitution-failed
A fair criticism.
Our Founders used it as a starting point and came up with just the right balance of strength- which balance was later destroyed in favor of the central government.
Might have something to do with this:
https://books.google.com/books?id=xY_RAAAAMAAJ&pg=PA413
“The inhabitants of Virginia were controlled by the central authority on a subject of still more vital importance to them and their posterity. Their halls of legislation had resounded with eloquence directed against the terrible plague of negro slavery. Again and again they had passed laws, restraining the importations of negroes from Africa; but their laws were disallowed. How to prevent them from protecting themselves against the increase of the overwhelming evil was debated by the King in Council, and on the tenth day of December, 1770, he issued an instruction, under his own hand, commanding the Governor, “upon pain of the highest displeasure, to assent to no law, by which the importation of slaves should be in any respect prohibited or obstructed.”