I think, if it’s really happening, one of the major contributions why christian community is dropping is because christian is a free religion. Meaning, it’s member are free if they would like to convert into other religion such as Islams, Hindus etc.
The Catholic church doesn’t condemn or set rules that if a member of Catholic faith will convert to other religion, that person will be punished or denied by the Church or his family.(am i right?)
Unlike for example in Islam, if you denied your Islamic religion you will be punished (some says even your family members have the right to kill you).
The Christian or Catholic faith is also not prohibiting it’s community to be married with other person with different religious faith. They are free to choose if they’d like to get married with Islamic, Buddist etc.
While in Islam or maybe other religions as well, once they marry non-islamic spouses, that person (husband/wife to be) should convert to Islam first, so that could lead to faster increase rate.