^^I'd imagine because we have Luke in the Mandoverse.
Of course it could be argued that we have Mark Hamill to draw from performance-wise, whereas in the case with Carrie Fisher, there is only the film-work she is involved with. It'd be difficult to provide more than a small cameo. The only time it happened beyond that was having a digitized Peter Cushing as Tarkin in Rogue One, which even then had a new SW actor Guy Henry saying the lines...but despite the film's critical success, the uncanny valley wasn't particularly a positive highlight. And that is with a character who isn't as beloved as Leia Organa.
Regardless of what direction they go, the new Leia may not satisfy everyone beyond animation.