Why should a church change its values? Because the membership wants it to, and they don’t want to lose that membership.
Churches are the focal point for communities. People born into Mormon families may not be believers, but they still might be Mormons. Why? Because it is the community. These are the people they know and have grown up with and have done everything with.
When your community shuns you for whatever reason, it is very painful. Many people go through all kinds of mental and spiritual contortions to find a way to stay with their community.
If enough people in the community develop values antithetical to the church’s values or the religion’s values, the religion has to choose whether to stay or go. The ordination of gay priests in the Episcopal church lead to a split where more conservative churches have tried to leave. They tried to take church property with them, but that’s another story.
The reason why the Episcopal church now allows the ordination of gays is because there were years of discussion ending with enough people supporting it that the church itself could incorporate that into canonical law. This can happen in any religion—even Catholicism changes, albeit a bit more slowly.