I can't explain it. The foot traffic in downtown San Francisco is growing. On Sunday evening the 30th of May the foot traffic on this corner (4th and Market) was greater than at the height of Christmas season. This evening, at 6:30 on a Tuesday, it was surprisingly heavy.
Several forces are clearly at work. One, this corner is important because it is one block away from the Mission garage and the Sony Metreon building, a regional destination hub. Two it is the gateway from the Mission garage, Bart and S.F. Muni system to the downtown shopping area.
The third reason is speculative. I keep finding evidence that tens of thousands of people from Fresno to Carmichael have apartments in San Francisco to use as a holiday shopping retreat. San Francisco is being hollowed out of worker residents and filled with regional shoppers.