Well, some people probably value privacy online. Especially if a relative is still alive. For example, my grandfather on my mom's side was alive when I registered here on Wikitree. He passed away a couple months ago and his page was still marked as private. I recently made it have the yellow lock.
But, if you are talking about people further back than that, then I have no idea. Why lock someone's page if they were born in the 1800s or something?
I can see people being private for security reasons mostly. The internet can be a scary place and some people don't want certain bits of info out there.