There are clues there:

1) they wanted everyone to be in once place 2) they're hiring a lot of people, especially customer service.

Customer service in NYC or SF would be prohibitively expensive, so if they want to be all in one place, they would have to move anyway. This is no different than Zappos being in Las Vegas or Groupon being in Chicago (I think Zappos relocated). Personnel costs when you have 50 employees per hacker become a big deal.

Portland has a ton of skilled, educated people and high unemployment, so they're going to get a lot of bank for their buck.