As English has become the world's dominant language, it has become easier for Americans to be influenced by foreigners who are fluent in English. For example, Americans follow political controversies in Iran by reading blogs by Iranians — Iranians who like to write in English, of course, which is hardly a representative sample of Iranian opinion.
This means that the American press will tend to be biased toward political movements who represent the better educated, wealthier, more cosmopolitan, Internet-savvy, and more elitist elements in a foreign country (i.e., those likely to speak English well), while the American media will be less sympathetic toward parties comprised of the less educated, poorer, more xenophobic, offline, and more populist elements.
Thus, the American media was sympathetic toward Mousavi's complaints about vote-counting in Iran because because his supporters were good at communicating them to Americans, while the populist Ahmadinejad draws his support from uncool people who don't speak much English. In contrast, the complaints of Lopez-Obrador, the populist mayor of Mexico City, about vote-counting in Mexico were greeted with yawns in the U.S. press because his supporters are generally not very articulate in English, and his party's ideology is fairly anti-American and anti-globalist.
Being biased toward the better English speakers is not just a custom of convenience for the American media and the American government. There's a moral feeling as well that the better English speakers deserve to win because they are more like us. Of course, this is self-serving: promoting the triumph of English-proficient classes also promotes the global dominance of American media institutions.
This is hardly a new phenomenon, of course. FDR's Administration routinely overestimated Chiang Kai-shek's regime in part because it possessed a facade of charming English-speaking UCLA and Berkeley-educated officials, even though the real decisions were made in very Chinese ways that Washington never understood. Meanwhile, Mao's rebels had few English speakers, so FDR underestimated them.
Similarly, why did the U.S. side with Maliki's Iranian-aligned Shi-ites in Iraq, when it would have made more strategic sense to side with the anti-Iranian Iraqi nationalist Shi-ites of Muqtada al-Sadr? A big reason is that Maliki's gang, who had spent decades in Iran while their rivals were holed up in the slums of Iraq (such as Sadr City), were more cosmopolitan — i.e., were better at speaking English.