I think its really a lack of space issue. There is no room to open an additional one and they would build a new one but the Salt Lake City Airport is suppose to build new terminals soon so i think its not worth the extra money. There is no doubt that DL will be SLC for the foreseeable future i think its just too much money for too short a time to build a second one. When the new terminals are built im sure this will go in the plans for a much larger one if not two.
Last i heard DL was pushing for the new terminals and the train service to be built. The train isn't paid for at all by delta but they supported it. The new terminals DL wants to be economical since SLC is "profitable right now" and has the lowest operating costs they don't want to see them go thru the roof with overly flashy new terminals.
Its such a shame that CVG, STL, and PIT have really nice terminals with no use and they can't be airlifted to SLC. CVG is going to mothball whole terminals with delta clubs if that could be airlifted it would be perfect in SLC. Concourse A at CVG is perfect for SLC with a nice delta club if it was at all possible to move it. I think concourse c and terminal 1 are also mothballed don't remember if they had delta clubs though.