The reformists had their day under former President Khatami, but his administration led to 2 factors that have totally changed the situation after that, and for the worse. Khatami was the golden boy of the reformers, who wanted to break the stronghold of the clergy over Iranian Government policy and direction. However, over the period of his administration, he was unable to create much space for himself and would frequently fail to overcome the rigidities of the clergy, including the Supreme Leader Ayatollah Ali Khamenei and the conservative Guardian Council. This eventually disillusioned the reformist section of society and led to their absenting themselves from the political process, thus playing into the hands of the conservative hardliners. At the time of the last election, the hardliners had enough of some of the efforts of the reformists and vowed to do what they could to reduce the effect of the reformists.
So, first the conservatives galvanized all their supporters and got many of them worried about the decline in the fervor of the Islamic revolution, leading to most of them turning up for voting. Secondly, since the conservatives controlled the prime levers of the power, including the Guardian Council (which okayed the nominations of candidates), they were able to prevent many reformers from standing for power. Together, these 2 factors reduced the power of the reformers: