I made mine private when it became clear to me that there are unresolved issues on my tree which I don't want people blindly copying.
Other people make their tree private when they get tired of people taking the info they've worked hard to find without even saying thank you.
Some people might become disillusioned ACOM altogether or found another site they prefer to host their data and therefore no longer want their data here.
The owner may have died and their family members didn't want the account/data to remain on ACOM.
There are probably many more possibilities.