Reasons for being denied are as varied as people applying.
Not disabled enough is common.
not providing paperwork, missing appointments, doctors do not cooperate, and provide documents. Cannot afford copies of medical records, etc.
The process is application, if denial, rehearing, if denial, hearing before administrative law judge, if denial, appeals counsel, if denial, US District Court, denial, Circuit Court of Appeals, if denial, US Supreme Court (which hears 1
SS case every 10 years or so).
Or it could be separate applications that are repeatedly denied.
No one has anyway to know in the abstract why he was denied without knowing his medical condition and the written decision by SSA.