I think it was because they had the best bid. They were the ones that were most committed to make things work.
Depends on what you mean by "best bid". If you mean , the most money to throw at technology, stadiums, infrastructure, etc. Bringing major football to a part of the world that rarely sees it and hardly qualifies more than a single team for the World Cup. Yes then Qatar had the best bid.
If you consider things like revenue for Fifa, match attendance, expansion of footie in a major market, amount of pressure (lack there of) on building new stadiums, hospitality services, etc. The the US had the best bid.
If Qatar 2022 goes as planned, it's going to be cool as in space-age futuristic cool.
The USA was a less risky and more certain bid, but as we've seen with South Africa the more risky bids can impress. Just need to make sure they test the ball next time.
Brazil 2014 should be interesting, considering they're already having difficulties.