U.S. Soccer, on the other hand, clearly feels that it wasn't in its interests to publicly oppose Blatter if it knew he was going to win. For one thing, there hasn't been nearly as much coverage of the FIFA election in the U.S. as in England, so the benefit of taking a stand wouldn't be as high for the USSF. And U.S. Soccer must feel that it still needs Blatter's FIFA to support (or, at the very least, not oppose) a number of things other than World Cup '22 as the sport continues its development in America.
What could those things be?
World Cup TV rights for 2018 and '22. In 2006, Blatter's FIFA did U.S. Soccer and MLS a favor by agreeing not to accept a $350 million bid for the TV rights for World Cups '10 and '14 from NBC, which had no interest in committing to showing MLS or other international games. Instead FIFA later accepted a $325 million bid from Univisión (Spanish) and a $100 million bid from ESPN (English), which agreed to increase soccer's domestic exposure by doing deals with MLS and U.S. Soccer as well.
The U.S. TV rights for World Cups '18 and '22 will be up for bidding in the not-too-distant future, and U.S. Soccer would prefer that Blatter's FIFA choose a partner (read: ESPN) that also has a commitment to MLS and U.S. Soccer.
Continued FIFA approval of MLS's unique aspects. If it was up to Blatter, MLS would never include more than 20 teams (it currently has 18 and plans to expand beyond 20) and would play a fall-to-spring schedule (instead of a summer slate). Blatter might also have issues with MLS's single-entity structure and lack of promotion and relegation, but FIFA hasn't tried to crack down on those things during the years that U.S. Soccer has supported Blatter. Without that support, things might change.
Keep in mind, too, that Blatter's FIFA did U.S. Soccer and MLS a big favor in 2009 by not intervening in collective-bargaining negotiations between MLS and its players, despite calls to do so from FIFPro, the international players union. If Blatter wanted to make life difficult for U.S. Soccer and MLS, FIFA could change its stance on that also.