You may recall 20-25 years ago he was either the lead or was supportive in shutting down Senate hearings on the fate of U.S. military personnel reportedly held captive by Russia beyond the end of WWI.
I recall at the time wondering in light of his family history and indeed his own VN war record, what could possibly be his motivation to deny the families of lost servicemen the opportunity to obtain information that would help to gain closure.
Now looking at his entire record, it seems clear your answer is the best answer.
Ross Perot hated McCain over his treatment of pow families.