There can be several reasons that restore points are delted.
Points older than 3 months are deleted
Points are delted if there is not enough space. An older point is deleted to make room for a new point
Usually, the cause is your software that is deleting the points, defrag will delte points, some antivirus software or maintenance programs.
Here is some literature that may help;
System Restore "restore points" are missing or deleted
System Protection - Change Disk Space Usage
System Restore Points Keep Getting Deleted