Dave Filoni, writer of the Ahsoka series, brought Lars Mikkelsen back as Thrawn in his live-action series. He previously voiced the character in Star Wars Rebels.
Speaking to Entertainment Tonight, Filoni dished on bringing back Mikkelsen. He called the decision to cast Mikkelsen in the first place “a very big challenge.” Though he claims that Mikkelsen was always in his head as a choice. “Coming to this [Ahsoka], talking to Jon [Favreau], I was like, ‘[It's] gonna be very hard to beat Lars because Lars is in my head. The sound of his voice is kind of what I write.'”
Other options were considered, but Filoni stuck to his guns and cast Mikkelsen.”I couldn't he happier,” he confessed. “He looks wonderful in blue. Not everybody does, so that's a factor.”
Jokes aside, it makes sense why Filoni wanted to bring Mikkelsen back as Thrawn in Ahsoka. However, some fans will note how that was not the case when it came to the titular character. Ashley Eckstein voiced Ahsoka beginning in The Clone Wars. Filoni created the character for that Star Wars series and Eckstein reprised the roles in her other appearances in Rebels, Forces of Destiny, and Tales of the Jedi.
Beginning in the second season of The Mandalorian, Rosario Dawson took on the Ahsoka mantle. She reprised the role in another live-action series, The Book of Boba Fett, before leading her own series. While fans loved Eckstein's voice acting work as the character, perhaps Disney and Lucasfilm saw Dawson as a bigger draw given her name. Lars Mikkelsen is another established actor prior to his Star Wars work. It makes more sense why Dave Filoni and Co. would want to bring him back into the fold.