Here is a graph of Utah birthrates over time:

https://gardner.utah.edu/wp-content/uploads/Utahs-Fertility-Rate-is-at-Historic-Low.png

As you can see the birthrates have been steadily declining over the last 50 years, and newer data than this picture shows that the birthrates have fallen below replacement.

In addition to this, from what I hear there is a mass exodus of young Mormons leaving the church and becoming non-religious. So Mormonism is shrinking very rapidly.

Why is this the case?