The 70s were the heyday for the American Kiruv movements for a few reasons that are no longer true
(1)Back then even secular American Jews had a very strong Jewish identity
(2) Back then there was more of a general turmoil and seeking in the world.
(3) Back then the truth of Torah M’Sinai was much more accepted by most Americans, Jew and non-Jew alike.
Over the past thirty years, however, the atheists have very become much more aggressive in the general America media and public life, which have become way more hostile to ALL religion (with the exception of Islam)
Hence the continual decline of American kiruv success
The above is only true about the US. In Israel, the BT rate is not really less today than it was twenty years ago