Is it important to pretend questions have not been addressed before?
First, since the end of the Apostles, revelation has stopped. What we have now is unfolding of what it already revealed.
Second, as has been said umpty-ump times, councils and new ‘definitions’ are made when called for — or IMHO, sometimes a little AFTER they are called for.
For both Marian dogma there was mounting popular pressure for a declaration, and there were even (we think) miracles. When Mary appears to Katherine Laboure as “conceived without sin,” — a couple of decades before the relevant encyclical then either the muckety mucks have to say the vision is bogus OR they have a little pressure toward saying, “Okay, she was conceived without sin.”
"In order that the full and living Gospel might always be preserved in the Church the apostles left bishops as their successors. They gave them "their own position of teaching authority." -Second Vatican Council.
"The Tradition that comes from the apostles makes progress in the Church, with the help of the Holy Spirit. There is a growth in insight into the realities and words that are being passed on." - Second Vatican Council.
"The value of the Fathers and Writers is this: that in the aggregate they demonstrate what the Church did and does not yet believe and teach. In the aggregate they provide a witness to the content of Tradition, that Tradition which itself is a vehicle for revelation." -The Faith of the Early Fathers.
"It (Tradition) comes through the contemplation and study of believers who ponder these things in their hearts. It comes from the intimate sense of spiritual realities which they experience." - Second Vatican Council.
So Mary's Assumption lay dormant for centuries, until it somehow springs to life in modern times through pious and infallible contemplation?