Very good question.
Multiple reasons actually (which are related to the multiple changes)...reasons that happened both almost simultaneously as well as sequentially.
#1...In the 1970s, the Lds upticked their number of converts from a Christian background:
Since most Mormon converts in the 1970's and 1980's were coming from a Christian background, it was becoming apparent to LDS leaders in the 1980's that ridiculing the Protestant minister in the temple film was offensive to many new converts. There were even some reports of converts attending the temple once, and vowing to never return -- sometimes even refusing to return to any LDS meetings.
Source: Why the temple ceremony was changed in 1990 - LDS-Mormon
What was happening is that during temple endowments (where Mormons learn the secret handshakes that get them into their Heaven), Protestant ministers were portrayed as hirelings of Satan. So this was obviously provocative; offensive to say the least.
#2...Whether there was a direct linkage or not, temple attendance was down into the mid-to-late 1980s.
#3..."In 1987, David John Buerger (an active but liberal Mormon), published an article in Dialogue: A Journal of Mormon Thought, a liberal Mormon publication which is not controlled by the LDS church. In the article, Buerger suggested that LDS church leaders needed to seriously consider making changes in the temple endowment ceremony to counter declining rates of attendance."
Source:
Source: Why the temple ceremony was changed in 1990 - LDS-Mormon
#4..."Although possibly just a coincidence, the Mormon Church issued a survey to about 3,400 members in Canada and the U.S. to determine members' opinions concerning temple work and various other topics only a few months after the 1987 Buerger article. Soon after the 1988 survey, plans were underway to change the endowment ceremony again (the ceremony had been modified many times since its introduction in Nauvoo, Illinois in the early 1840's)."
Source: Why the temple ceremony was changed in 1990 - LDS-Mormon
So, besides the Protestant-ministers-as-hirelings offensive portion removed, what were the other four primary changes?
a. All penalties (and gestures like throat slashing, chest slashing and bowel slashing) were eliminated.
Obviously, 'twas overboard; not very "P.R." oriented; too violent in an increasingly violent late 1980s culture; and too obviously both pagan and Masonic riddled.
b. The strange words "Pay Lay Ale" (meaning "Oh God hear the words of my mouth") were eliminated. Ed Decker says the actual direct meaning of the Hebrew words ("Pele Heylel") is "Marvelous Lucifer"
Source: Is Lucifer the god of the Mormon Temple? [The OTHER World Series]
c. Women's promise to be obedient to husbands was modified.
Ya gotta remember the full emergence of the feminist movement then.
d. The intimate position at the veil (foot to foot, knee to knee, breast to breast, hand on shoulder and mouth to ear) was eliminated.
Yup. I'm sure potential "sexual harassment" issues figured into this change.
Ya just gotta wonder: Did God change his mind about pagan-like rituals? Masonic-riddled rituals?
I know some Web sites exist recording all the ritual changes made periodically. This one did ... but is now defunct: http://www.lds-temple.org/
What if you work your way up into the Mormon church, get your TR, and then start producing YouTube videos of the secrets? What kinds of sanctions or discipline will they throw at you?
Another reason for the changes that isn’t often mentioned was the “Pace Memo” relating to the Satanic Ritual Abuse scare of the early 1990’s. After a 30 month investigation by the Utah AG’s office there was no evidence found. However, about the same time as the changes, Glenn Pace (an LDS General Authority) issued an internal LDS memo that sugguests that the victims had flashbacks when going through the LDS endowment ceremony for the first time which is in part what led to the changes.
The LDS endowment is hyped (even on this thread) as being sacred, but reality is it is silly. Members are constantly told that the temple is the most sacred and most spiritual rituals they will go through - they are primed for this ‘very spiritual experience’, yet most ex-mos admit that their first reaction going through for the first time was “WTH????”.
Here are some links, one to the text of the LDS temple ceremonies now with links to historical ones, one about the Pace Memo written while the AG investigation was just starting (and a huge topic of the news in Utah at the time), and several about peoples reaction to the LDS endowment ceremony.
http://www.utlm.org/newsletters/no80.htm
http://www.exmormon.org/mormon/mormon243.htm
http://www.exmormon.org/mormon/mormon285.htm
http://www.exmormon.org/mormon/mormon523.htm
http://www.exmormon.org/mormon/mormon564.htm
http://www.exmormon.org/mormon/mormon033.htm
~ Wilford Woodruff, 4th LDS President