They didn’t want to use a different voice actor for Ackbar after Erik Bauersfeld, who originally voiced him in ROTJ & TFA, died. Which is why he only had one line which was recorded at the time of TFA. I fully respect their decision on this.
They recast Peter Mayhew, they refilled R2 after Kenny Baker’s passing, a new muscle man wore the Vader suit, and they even brought back Peter Cushing. Life goes on, and having another voice artist provide the vocals for Admiral Ackbar would have meant no dishonour to the late Erik Bauersfeld.