One could answer by looking at their list: proximity to international airport, mass transit, regional population, ...
However, some are saying that Amazon was actually looking for DATA. Here are two articles:
- Bloomberg: Amazon Said No to Cactus, Yes to Data in Hunt for New Office
- Reason: HQ2: How Amazon Made Government Do Their Bidding for Free (note: this article links to the previous one)
Neither article is very long so you should read them yourself, but I'll provide a couple of interesting quotes:
From Bloomberg "But it kept hundreds of millions of dollars worth of free information from the cities to create the biggest corporate site location database in the world, according to Richard Florida, an urban studies professor at the University of Toronto."
From Reason: "Amazon is now privy to information about where different municipalities are going to direct investment and infrastructure in the near future. The company can exploit this information. ... Maybe Amazon just happens to purchase a new fulfillment center right around a soon-to-be-developed locale which would see increased demand for Amazon products. Maybe it simply decides to squat on land for a while, knowing that it will soon be smack dab in a hive of activity. A new brick-and-mortar store? They'll have the option. Or maybe knowing where news roads will be built will make it easier for Amazon to plan transit routes. There's profit to be extracted from this data that you and I could not even conceive."
Whether Amazon played a game just to obtain data or the data is a side benefit of an honest search, it's clear that data matters.
By the way - while not the same level and volume of data that Amazon got, ALL of us have access to a great deal of government data for free. Check out IPUMS.