There are a lot of factors. A big one is issues of privacy and consent from the other people involved. I can understand why some parents (especially ones who have grown up with social media) might think about the effect on a kid if they some day read blogs about their upbringing.

While I don't have that specific consideration, there are family-related things I never blog about because family members read my blog(s) and even if not, people who read my blog(s) know some of my family members personally.