Why didn’t the LDS church shut down the bill? I have no doubt that they don’t like this and they have enough sway to order any Mormon legislator to vote it down or face excommunication.
My suspicion: the LDS Church has enough trouble keeping people who would leave if they could not be Mormons if homosexual. People can choose their religious affiliation, but they do not choose being straight or homosexual.
Conversion therapy is psychological and medical malpractice. Mormonism is considered a cult in some parts of American life, and Mormons probably think that the sort of conversion therapy used against gays and lesbians could be used against someone who becomes a Mormon elsewhere.