I hate to go against the flow here on Kos, there were probably some irregularities in the vote, and yes the result is not as we would have liked.
However how much fraud went on is debatable and how the vote really was split will probably not be known.
Mousavi was prime minister [1981-89] right at the start of the cultural revolution and indeed was responsible for the putting in place of many of the Theocratic Regimes most repressive policies. In fact he oversaw 30,000 political executions during 1988. His party was also known for many sexual scandals at the time.
Hence I find it hard to believe he would have overwhelming popularity
My opinion is that he saw an opportunity to reignite his political career and he saw real unrest within Iran. He already did this at the start of the cultural revolution and was indeed head of a socialist party who then implemented the Ayatollahs policies. How much he actually believes in reform is now a moot point.