I think it's simply because this approach is more prone to abuse. A good dictator leaves democracy in the dust in terms of efficiency, but converesly a bad dictator can make everything go to shit at moment's notice. In economy/politics, it's a matter of life and death. In open-source software, the risks of dictatorship don't matter - someone can always fork the project off and do it better, but even if not, no one is starving or dying because of it.
Seriously, the amount of basic misunderstanding of all things not programming related here on HN is starting to get to me. I can't read these threads without being compelled to correct such obvious errors. If you don't know what you're talking about, at least phrase it in a way so that it comes across as an opinion, or better yet, research before you type. It's ruining the quality of these threads.