Probing and other things. I don't doubt that you could give extended answers but that doesn't change anything, replying with text gives you time to think about your answer, in some cases changing what you would say in an oral conversation. Not to mention that the way people speak and the way people write differ, that makes quotes taken from the written interview really stand out. Text also takes the emotions away, the way you say things is important - ideally, it'd be the best to to do face to face interviews but that requires time, funds and base of willing interviewees I don't have.