[StrictDabbler]

There's no single cause. Forming memories requires many parts of the brain. Injury to or illness in any one of them can cause anteretrograde amnesia.

It's like asking "what makes a person unable to walk?" Arthritis, paralysis, muscle wasting, MS, Parkinson's, a broken bone, an amputated foot... some are temporary, some are permanent.

Walking is hard, even though most of us can do it. Forming memories is similarly hard.