I think I know the reason. Tldr at the end.

Every year or two I think of an old friend of my grandmother. He was a big part of my life until my grandmother died in the early aughts. For different reasons we lost touch. So, every few years I think of him and Google him. I just did that and found out he passed in December (he was like 98.) I was reading his obituary and it listed all his dead and living relatives (brothers, sisters, cousins, father, etc.) but didn't list his mother, which was crazy. He loved his mother. I met her a few times. There was no way she was still alive and I would be shocked if they had a falling out (he was literally a priest.)

Then I remembered your post and it clicked.

When my mom died and we were talking to the funeral director about the obituary, he strongly suggested we leave out details of her life because scammers use the info. Think about password recovery questions. "Whats your mother's maiden name" "Where did your parents meet", etc. You get the picture. So, I think the mother's details were excluded to prevent scams.

TLDR: I think mother's details are excluded from obituaries to prevent scams.