VCs have longer time-horizons (a typical VC fund will be spent over 5 or so years) and appear more stable to landlords. In addition, one of the toughest problems for VCs as I understand is dealflow - when it comes to a super-competitive round, Andreessen or Sequoia will often push other VC firms out of the round by throwing their prestige around.
SF Office space could make the difference to a hot start-up looking to raise their Seed or Series A and move to the city, especially coupled with a time horizon of only 12-24 months. It may not be enough to lead a competitive round, but may certainly enough to get a piece of it.