Limbo (which is what I think you are referring to, instead of purgatory, which was and still is an official teaching) was never part of the church doctrine:

The verdict that limbo could now rest in peace had been expected for years. The document was seen as most likely the final word since limbo was never part of Church doctrine, even though it was taught to Catholics well into the 20th century.

Source: https://www.reuters.com/article/us-pope-limbo-idUSL2028721620070420

More quotes from the article:

Limbo, which comes from the Latin word meaning “border” or “edge”, was considered by medieval theologians to be a state or place reserved for the unbaptized dead, including good people who lived before the coming of Christ. (...)

In writings before his election as Pope in 2005, the then Cardinal Joseph Ratzinger made it clear he believed the concept of limbo should be abandoned because it was “only a theological hypothesis” and “never a defined truth of faith”.

In the Divine Comedy, Dante placed virtuous pagans and great classical philosophers, including Plato and Socrates, in limbo. The Catholic Church’s official catechism, issued in 1992 after decades of work, dropped the mention of limbo.

The official cathecism gathering all the official teachings was only issued in 1992.

Basically, limbo was a drafted theory that was taught casually but never an official teaching, and the idea behind not endorsing it and clarifying it when the cathecism was written, to justify not including it there, I suppose, is that limbo is not necessary.

“There is greater theological awareness today that God is merciful and wants all human beings to be saved. Grace has priority over sin, and the exclusion of innocent babies from heaven does not seem to reflect Christ’s special love for the little ones.”

This one quoted from here