Diego passed up the chance to run for Mexico last year to leave open the possibility to run for the US. See:
http://www.runningtimes.com/Article.aspx?ArticleID=24969&PageNum=2
"Then, an impressive track season in 2011 left Estrada at a crossroads. He'd run the world championships "B" qualifying time for the 5,000m, and Mexico offered him the opportunity to compete in Daegu. Estrada was torn, saying, "I was looking forward to representing Mexico at a world champs, but when I got selected for Daegu, I realized that if I took that opportunity, I would have to sit out three years before I could represent the U.S. That's when I took a step back and decided to [decline Mexico's offer to run in Daegu] and start my citizenship process." Estrada became a U.S. citizen on Oct. 28."
So once he thought he couldn't run for the US this year and Mexico said you can run at the Olympics, it made sense to pursue that. Its impossible to say what would have happened if he had received accurate info from the start.