"I don't see how he could have failed a medical," Redknapp said on Monday. (Agencies)
"He had a stringent one here, one at Marseille and at Newcastle and with France before the World Cup. You couldn't meet a fitter lad. There has to be another reason," he added.
Remy, who scored 14 goals in 26 league appearances for Newcastle last season, will start the new Premier League season at QPR.
"Their loss is our gain - we've got a fantastic striker," Redknapp added.
"I said last year he was capable of scoring 20 goals in the Premier League. That's what we now want him to do for us,” he said.
The 27-year-old joined Liverpool on tour in the United States to complete his 8.5 million pounds (USD 14.44 million) move but the deal fell through on Sunday.
Reds manager Brendan Rodgers told reporters: "It's very simple - we have made a decision as a club not to go ahead with the deal. It's unfortunate for the player. But there's nothing more to be said."
Remy moved to QPR from Marseille in a club-record eight million pounds (USD 13.59 million) deal in January 2013 and joined Newcastle on a season-long loan last August following QPR's relegation to the Championship.
"I don't see how he could have failed a medical," Redknapp said on Monday.