Keep in mind that B6 launched SLC-SAN with SLC-SFO. Later, BUR and LAS were added as well. But with only one frequency on each route (a second summer seasonal to SAN was added for 2008, but didn't return this summer), it was hard for B6 to compete with DL's multiple frequencies.

I think B6 originally wanted to use SLC as a mini focus city with multiple purposes, including increased gate utilization (which would thereby lower average cost per flight). But that obviously didn't work out, and I think B6's focus has now shifted to more profitable places (namely Caribbean/South America) and to defending its turf (BOS).

As far as SAN goes, it was the best performer out of SLC, but still was not up to par with B6's overall system. Low yields and low load factors are not going to help anyone.