I have to admit its a puzzle - SAN is a pretty nice station to visit. I've gone through there a few times and been pleased. Check in was good - they maintained Medallion service even when other stations were trying to divert Medallions to the Kiosks last year. Elite Security worked fine. Its got a small but nice SC with personable staff. On one flt the pilots let my kid come up pre-flt and let him wear the captains cap while my wife took a pic.

About the only thing I can think of is that SAN fares are a bit high but not really out of line with DL's tendency to overprice this summer.