They can't. The main reason is Mako the actor who voiced Aku sadly passed away in 2006 . If they got a different actor to play him it would be just a guy either trying to copy his voice or another big named actor who just isn't the same . Mako was Aku plain and simple . It would be disrespectful to get an actor to replace him.
Well, they had to replace him on Avatar: TLA since he was Uncle Iroh. Maybe they could just reuse clips of his voice from the show and just have Aku show up at the very end?
Well I feel it's disrespectful·Just using sound clips means he wouldn't be able to do anything and what made Aku so great is his dialog · Maybe if they get someone who can immigrate him rather well while still giving him personality like Jim Cummings or Jeff Benenet
Well, they had to replace him on Avatar: TLA since he was Uncle Iroh. Maybe they could just reuse clips of his voice from the show and just have Aku show up at the very end?