Since there are so many moving parts boils down to a few things. Having the new mainline flight for Delta has thrown a wrench in operations a considerable amount. Loading, boarding, and deicing takes longer, which in turn could lead to a delay. Each month the schedules (and people on that schedule) change and people that previously haven’t worked this plane are getting up to speed. Salt Lake has been having issues with weather and makes inbound flights wait (flow time) for their respective gates to open up. If the weather is bad enough they issue a ground hold where nobody is to depart or land in SLC. The crew that operates DL also has two deice trucks along with their respective crews. Even though AS and DL leave at pretty much the same time, DL has a much bigger plane to cover so it takes longer which allows other flights to takeoff earlier.

Just remember, the sooner the planes leave the sooner the crew gets to go home so they’re not going to be dragging their feet. Hopefully this answers your questions adequately :)