I went to Catholic school, and we had a joke that Catholic school creates more atheists and agnostics than any other institution.
But the truth is, most previous Catholics still have some kind of attachment to the religion. Either we still say a prayer once in a while, or we still like sitting in an empty church once in a while, or we still like the smell of the overwhelming incense they used when we were children, or we find Protestants particularly annoying. I think old religions die hard. Belief sets connect with us at some emotional memory level that I think is extremely difficult to disconnect from.