To prove that the Seattle Commons park could actually be built, Paul Allen acquired, or acquired rights to, a ton of land in South Lake Union. When voters rejected the property tax levy, he sat on it (as http://www.vulcanrealestate.com/). Between 2005-2012, he built what became Amazon's headquarters (http://www.seattletimes.com/business/amazon-to-make-giant-mo...) and a lot of Amazon's growth.
Paul Allen was pushing for the Seattle Commons concept. When it didn't pass, he wound up owning a bunch of land that had no likely tenants. What South Lake Union has become is his "Plan B" because it's his property development company (Vulcan) that has done the lion's share of the building.