I'd say it's because the Mormon Church is about trying to control politics. They want to own the system. I don't see their vision really being all that different than Trumps, just that they don't think he can win and are scared of losing the hold of their common interest. Evangelicals and Mormons oppose each other on religious doctrine but their societal agendas and goals are pretty much the same. The Mormon Church is just more pragmatic and tactical in achieving those goals, while Evangelicals tend to be more idealistic to the point of cutting their nose to spite their face.
That may be true. Yet they preach the same bigoted fire and brimstone vitriol. I grew up in the area the religion was founded and have experience with them too. You don't want to get on the LDS bad side. They can be very dangerous. The LDS is just like the the extreme crazy side of Evangelicals mixed with Scientology tactics. They are a scarier bunch IMHO. Very two faced.