The UN often has multiple conventions on the same thing (example). Is there any reason they do this rather than amending a previous convention? e.g. maybe it is harder to make an amendment than a convention, maybe amendments have less power than conventions, tradition, etc.
After the initial round of negotiations, nations sign and ratify a convention one by one. They would have to sign and ratify the amendment again. So there would be non-member states, members of the original convention but not the amendment, and members of both the original convention and the amendment. After a few rounds of amendments it would be difficult to understand which version applies to which country, and the use of precedent for interpretation would be difficult.