Most of the reasons the situation in St. Louis became a big news story was because of the protests. If you remember, things didn't "boil over" until several days after the actual shooting. With this shooting, there hasn't been mass protests every day for a week.
Secondly, in this situation, a group of officers of mixed races were present when the incident occurred.
Also, the Salt Lake police seem to have a grasp on how to handle things. Their chief is a huge proponent or officers wearing cameras and several different investigations of the incident by various groups are taking place.
Whether you feel the St. Louis situation was fueled by the race card or not, the handling of the situation by the police was pretty terrible, I haven't seen anyone that's not hellbent on politics not agree with that. Even the state police came in and were critical of the handling.
So much of what becomes news has to do with PR. If you handle a situation well, it won't be talked about. If you fumble the handling and create more controversy, it will be talked about more. Salt Lake has handled their situation well, St. Louis did not.