It's mostly because they are trying to curb the power of the HRE emperor (especially because the empire was united with Spain under Charles V). Same deal as to why France intervened in the 30 years' war on the side of the mostly Protestant opposition (including Protestant German Princes, Sweden) against the mostly Catholic HRE and Spain.

In terms of European international relations of that period, France value the weakening of the Habsburgs more than they value Catholicism or Catholic solidarity.