My only question is why did 3 countries that could easily (and have, in the past) host the World Cup on their own gang together? Mexico, Canada, and the United States are wealthy countries. Why gang up?

My guess is that wealthier countries are going to have to do a lot more cooperating if they want to host world-level events from now on, due to the fact that the selection process for these types of events has become democratized. Economist Branko Milanovic has a thoughtful piece on FIFA (the governing body for world-level soccer events) and corruption that ties in to all of this.