Why are they not "so definitive"? Definitive, dogmatic teachings are pronounced "de cathedra" only. Apart from that there are different positions that are taken in different time-frames and that are open to several different understandings.
We have a lot of things in "earlier Papal declarations" that are not deemed as correct today, especially when viewed in absolute terms and withouth taking into account the context of the times. This isn't to say that there was a "change" to the opposite, merely that the views on the same issue, while being reflected many times in the same formulation, changed by expanding on the interpretation of the specific formulation.
As such there is nothing "definitive" per se in the previous Papal declarations used to support Extra Ecclesiam Nulla Salus, which isn't to say that it isn't "right". Better yet: what several theologists consider to fall within that teaching is different, given that it depends on how one defines the Universal Church. This is something that goes back to at least Saint Justin.
In short, while there are merits in reaffirming the teaching, especially considering Modernism and relativism, I do not agree that in strictly theological terms the issue is so clear-cut.
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