Tim Burke has an interesting post up comparing Zimbabwe and Iran and the similar difficulties faced by the reformist movements in the two countries. For the detail and nuance of his comparison you'll need to read Tim's whole post, but the key elements of it have to do with the social constituencies supporting these movements for reform, on the one hand, and the determination of the 'securocrats' to stay in power, on the other; they have, Tim says, 'the means, lack of scruples and competency to do so, perhaps indefinitely'. I wonder about 'indefinitely', especially as this applies to Iran, but I don't know enough about the country to contest the issue. I will only say, as a general point, that the success of successful democratic movements can sometimes seem distant even on the eve of a major breakthrough.
What I find doubtful about Tim's argument is not the comparative analysis of Iran and Zimbabwe; it is, rather, the reflections in which that analysis is encased, concerning the (relative) silence of liberals and the left about what is going on in the two countries. He attributes it in some part to a sense of futility and impotence in that political quarter, faced with the internal obstacles to successful change identified in the analysis he offers. He even turns this suggestion against some who have been vocal, describing their non-silence in less than flattering terms.
Why any particular person - journalist, blogger, citizen - chooses to speak and/or act about one issue rather than another in the densely abundant politics of our planet is affected by all sorts of contingent circumstances, to do with their outlook, their geographical location, their interests, their personal history, and so on. Moreover, given the large number of problems claiming the moral attention of all of us, no one is obliged to be active about this problem rather than that one, or is delinquent for not having anything to say about an issue that you may happen to highlight for them - Zimbabwe and Iran being two such issues. Each person assigns their political energies as they may.
However, when it comes to large-scale trends it is not merely accidental what issues people gravitate towards in significant numbers. The contemporary liberal-left is much more drawn towards some of these than to others for its condemnation and protest - the actions of Israel, the US and other Western democracies come to mind - and there are political preferences and blind spots involved in this. What I doubt, as a matter of sociological explanation, is that their choices have to do mainly with the prospect of success. Or, looking back: the anti-Apartheid struggle did not, at the time, seem like one that would be rewarded with an easy or proximate victory, but this deterred no one from speaking and acting in solidarity with the South Africans waging it. One may or may not want to get involved oneself, but those who protest from afar in support of the democratic movements in Zimbabwe and Iran do something of what they can do. Silence as the better option for such situations is an eccentric and paradoxical counsel within the traditions of progressive politics - though naturally, as with everything else, one can speak and act to better or to worse effect.