The All Blacks went out of their way to include the injured Waisake Naholo in the squad for the tournament, leaving in-form utility Piutau on the sidelines.
Piutau told the Daily Mail that All Blacks coach Steve Hansen told him his move to Ireland was not the reason for his omission, but the former Blues back remains unconvinced.
"It's human to have those 'what ifs'," he said.
"I gave everything I could to be selected. I knew there was a possibility that moving could affect my chances.
"The coaches told me it wasn't the reason, but I felt it was, partly. It was tough. No player likes to be a spectator in the biggest tournament.
"It was quite hard to swallow. I backed myself to make that team but it wasn't to be."
New Zealand Rugby were angered by the 24-year-old's decision to head for Europe before his test career had fully blossomed. NZR even blocked him from playing for the Blues for the first part of the 2016 season, and he is turning out for English club Wasps before taking up the Ulster contract.