Why does sample size not matter in qualitative research? Because of the assumptions that qualitative researchers make, namely, that the social world is not predictable. Qualitative researchers believe that people are not like molecules or other objects; people’s actions are not predictable.
But quantitative researchers DO believe that social activity IS predictable. So when they compare their observations of social activity to what would happen in purely random results, the difference says something. Let’s say you were to research people’s preferences for a particular interactive feature. Say you’re wondering if young people will like a radio button more than older people. First, you model what results you’d expect if you’d just flipped a coin. Then you use a probability (random) sample, and compare those results to purely random results. Is there a difference?
If there is a difference between them, you can infer that indeed, something other than chance (in this case, age) affect people’s preferences.
Qualitative researchers don’t agree that such things can be reliably predicted. That’s why they don’t bother with expensive and involved random sampling. See all these details below from my research design course.