From the book, Preparing to Enter the Holy Temple, published by the Church of Jesus Christ of Latter-day Saints as a course of study for those who are about to receive temple ordinances.
Explanations for this policy center on the extremely sacred nature of temple ceremonies. First that something so sacred shouldn’t be exposed to public criticism and ridicule. Second that unless one has reached certain level of religious education and spiritual preparedness you won’t be able to fully understand the ceremonies.
Even within the church unless you have been involved in a some of the temple ceremonies you won’t know exactly what goes on. For example I was raised in the church and was a good Mormon girl up till the age of 17 or so I don’t know that much about it. I was told in sunday school that there is an altar and that you and your intended kneel across from each other. You make certain vows to your spouse and to God. And that’s pretty much what I know other than general doctrinal principles of why a temple marriage is important. If you’re preparing to get married in the temple or take out your endowments without getting married there’s a process of study and personal preparation that I believe includes getting some more information about the ceremonies but I never got that far.
This secrecy makes it difficult to write a good article about temple ceremonies because a devout Mormon who’s been through it won’t give you lots of specifics. So for first hand accounts you typically have to ask ex-Mormons, many of whom have reasons to dislike the church and that tends to color their narratives.