Why isn't Pope Leo XIII a candidate for sainthood?

Well…What does it change? We’re talking about the catholic church view, not about the US ot Italy’s views.

So? It doesn’t make their views erroneous from a religious point of view. Religious views aren’t supposed to be based on what is popular, or on the constitutionnal principles of such or such countries. Of course it happens that churches (and in particular the catholic church) adapt their teachings to the stance curently prevailing to some extent but they don’t have to do so.
You can’t state that the church’s position is erroneous because it conflicts with your own values or the values generally accepted in your country. The church teachings just have to be internally consistent. They can only be erroneous (from a catholic point of view) if they conflict with other catholic teachings. You’re not obligated to like them, but the church isn’t under any obligation to take into account principles that aren’t derived from its theology, either. It’s erroneous if it conflicts with the bible and the church tradition, not with the US constitution.