Why not me or my family?

Random selection as far as I know. I’d love to do it. Both our daughters have, one of them twice!

My understanding is that it's random. My mother and aunt have been called, but I've never been, nor do I know anyone else who has been.

I believe that jury duty is relatively uncommon because most cases are disposed of without trial (e.g. defendant agrees to a plea bargain, meaning that there is no requirement for a trial).

My son has been called, but no one else in the family

No, it’s random. I have only ever been called once, but a friend has done two. Some people are never called. I have a large extended family, and none of them apart from me has ever been called.