The international community put a lot of effort into getting elections underway in Afghanistan, and the momentum was likely a contributing factor to the high turnout. Also, I should mention that some limited experience with democratic reform in Afghanistan occurred through the 1960s and 1970s with the experience of the development of constitutional monarchy. There were limited parliamentary elections. All the gains made under those reforms have since been washed away. But there is some experience there, so democracy isn't necessarily a foreign concept in Afghanistan; it's just that it faces huge hurdles to becoming solidified.
Robert Dahl is the eminent democratic theorist in political science. He distinguishes between normative democracy and procedural democracy. Sometimes we might get tricked a little bit by thinking that countries have undergone a democratic reform, but the roots of democracy haven't necessarily taken hold and what we see is procedural elections and these sorts of things. It takes time for the seeds of democracy to take root in a more normative or meaningful sense.
In Taiwan, for example, the ROC initially allowed local elections to take place on a limited basis. Eventually it came to mayoral elections and then finally to presidential elections, but that whole process took over 50 years. And admittedly Taiwan had more favourable circumstances than Afghanistan does today.