Suppose, you are requested to arrange a few names alphabetically. Now I give you names one by one. When you get the first name you can't put it in an arranged way because you don't have the reference to compare the name with; where you should place that name. The arrangement approach is only be possible when you know/have all the names. Similarly, qlikview loading records are like getting data one by one. So qlikview would only be able to sort them when all values are loaded/available in the memory.
I guess, alternate could have been possible, but that could be costly in terms of memory utilization, hence QlikView avoids that.