The Maroons found themselves on the back foot for much of the early going in the game despite the fact Fifita scored the opening try.
He was taken from the field after just 21 minutes and didn't return until the game's final ten minutes.
Slater said that Fifita looking fatigued had caused him to come off the field in the first place before the eight interchange cap per game forced him to stay off.
"We just felt that he [David Fifita] was looking a little fatigued out there. We were doing a lot of defence and we just felt it was the right move to put Reuben on that edge," Slater said.
"He moves really well, Reuben [Cotter], defensively, so we could give Dave a spell.
"When you have eight interchanges, it's really hard. In hindsight, it's really hard to look back and go 'well, we'd love him out there more', but you can only have eight interchanges, and we spent that last ten minutes knowing we didn't have an interchange up our sleeves, so we had to roll the dice."
In his place, Reuben Cotter once again played in the back row.
The North Queensland Cowboys star, in just his fourth Origin and first full series after being injured last year, took out the Wally Lewis Medal for Player of the Series after playing all 240 minutes.
His Game 1 effort on the edge after an early injury will go down in Queensland history, while his performances across Game 2 and 3 were equally strong.
Slater said it was incredible.
"Incredible isn't it?" Slater asked.
"He only played one game last year and he got the man of the match. Now he has played four State of Origin games for Queensland and he has got a Wally Lewis Medal to put in his cabinet.
"He is an incredible player.
"I and the team value what he brings to our footy team. If every team had a Reuben Cotter, they'd be in a much better position."
Cotter becomes the second Queensland forward to win the award in two years, following in the footsteps of Patrick Carrigan, who took the medal last year.
The last back to win the Wally Lewis Medal was Tom Trbojevic in 2021.