To rightly divide" this fact, one ha to understand a little how the Catholic Church works. And this usually involves shedding the notion of a top-down organization where the top churns out doctrinal precepts and those down the line salute and say, "Yessir, uh, Father, sir."
Whatever you think of the Marian dogmata, it's important to get that one was 'defined' in the middle of the 19th century and the other in the middle of the 20th. Yet they had been discussed and argued and believed by many for centuries before their "definition."
It was only when the Holy See perceived a clamor that it turned the theologians loose with a mandate to come up with some direction. In other words, a with Acts 15, Nicea, Ephesus, Chalcedon, Nicea II, and many other councils, an "official" declaration was made in response to conflict or (in the case of the Marian dogmata) loud appeals.
So the delay in closing the OT canon was because there wasn't a pressing need until a whole bunch of people closed it themselves.
Come to think of it, the closing of the NT canon, as I was taught in my not-Catholic seminary, was similarly brought about. Marcion did his own 'sua sponte' closing, so the rest of the Church said, more or less, "Oh Darn, now we're going to have to resolve this somehow."
It's not like there wasn't an opinion, it's just that we don't like to get all official and stuff unless we have to.
Still it would clearly obvious from the very fact that they had to “debate the matter” that there had been no clear consensus up until that time and debate or differing with the “decision” was not an option after that time for fear of reprimand or excommunication.