Mainly because the hierarchy of the LDS Church has been actively working against marriage equality for 11 years, exposes the Bay Area Reporter:
The March 4, 1997 memo from Loren C. Dunn to church Elder M. Russell Ballard, describes an insidious “go ahead” HLM (homosexual legalized marriage) strategy of the late Mormon Church president, Gordon B. Hinckley. The memo appears to be tangible proof that Mormon leaders had been working to prevent marriage equality in California and Hawaii at least for the last 11 years while actively working to recruit the support of local California Catholic bishops…
In addition to revealing a long-standing Mormon strategy designed to recruit Catholic bishops to their cause three years before the state’s first political fight, Proposition 22, the memo also lists one of the Mormon’s key strategists as Richard “Dick” Wirthlin, a relative of Joseph Robb and Robin Wirthlin, the Massachusetts couple who were featured in Proposition 8 campaign ads this fall as the parents of a boy who was read a book about a same-sex couple in his second grade class….
Parents in the Lexington School District in Massachusetts disputed many of the Wirthlins claims to the B.A.R., pointing out that when the Wirthlins moved into the district they were already involved with two groups seeking to ban same-sex marriage. One of those groups, MassResistance, run by Brian Camenker, has been called an “anti-gay hate group” by the Southern Poverty Law Center.
Read the internal Mormon memo discussed in the article.