Who's trying to kill Utah's delegate/convention system in favor of primaries? Follow the money. Conventions are cheap, meaning candidates don't have to sell their souls to run for office. That's good for them, and good for us.
The primary system being pushed on us will cost candidates a lot. That's good for those who buy and sell politicians, but not so good for us. So who are these people that buy candidates and define an honest politician as "once bought he stays bought"? That would include special interests, big business and lobbyists. Who sells candidates? That would be professional campaign staff, the media's advertising outlets and direct marketers.
All of which explains why it's those profiteers, and not the public, who are pushing for the change.