It's because McCain sold out the missing POWS in Vietnam. Him and Kerry went on a 'government controlled/conducted' tour to look for POW's. Then, they came back, signed a document stating that there were no POW's left in prisons, and somehow John Kerry's brother was allowed to be the real estate dealer for a bunch of Vietnamese property (as soon as the ink was dry).
McCain and the POW Cover-up (How McCain left U.S. POWs and MIAs to rot in hell)
Thanks for the reminder & link.