I think it is because the modern world and the Catholic Church have different measures of progress - i.e, different yardsticks by which we show whether the world is getting better or worse.
In the modern world, convenience; plenitude; an increase of ability or power; an increase of information about the world; diversity of ideas; among other things, are considered measures of a better society.
Most (if not all) of these criteria of progress are either irrelevant or contradictory to the Catholic sense of progress. And when Catholics try to show the Church is progressive by the world’s metrics, it naturally fails because many of these metrics are at odds with the Faith.
For instance, it seems to me it would be wrong to ask whether the Catholic Church is more intellectually diverse than the world. The answer is obviously not; you have lost the faith if you hold certain positions, such as atheism. But that is because Truth is more important to us than a diversity of opinions.
So it seems to me the better discussions would be to question the secularist as to why we should care about his rubrics.
Is there any reason to care about non-Catholic measures of authority, progress, goodness, etc?
Would arguments be more compelling if they attacked the authority behind, or pointed out the existential irrelevance of, the modern world’s morals?