The answer is the obvious one.

It's that they've been pretty much making it up the whole time. [Along with most of the rest of it.]

Via the BBC: How can limbo just be abolished?

The Pope may be about to abolish the notion of limbo, the halfway house between heaven and hell, inhabited by unbaptised infants. Is it really that simple?

Pope Benedict XVI's anticipated pronouncement on limbo will have been informed by the International Theological Commission - a group of leading Roman Catholic theologians who have been meeting to consider the issue.

The Pope, himself, has been quoted in the past as saying that he would let the idea of limbo "drop, since it has always been only a theological hypothesis".