Once, when the Magisterium canonized the entire Bible declaring it inerrant and divinely inspired, wasn't enough?
No, it is not enough for Catholics, for whom Scripture is not the supreme or determinative authority, even after Rome finally provided an infallible, indisputable settled canon in the 16th century, and yet that was not the point.
For as you must know, affirming a source to be Divine, and declaring what it means are two different things, and which was the issue, and that the parameters of magisterial teaching leaves RCs with a great amount of liberty to interpret Scripture to support traditions of Rome as they understand them, (even if they do not rest upon the weight of Scriptural warrant).