Why was Tarkin CG, but not Mon Mothma?
it only very very briefly touches on it in that article: why didn't they just recast the part? Does anyone have a link to a relevant interview or article?
Adherence to strict continuity. Probably the same reason they re-used footage of Red & Gold Leader.
Jedi's like, 3 or 4 years off, so that gives them some wiggle room to cast a woman who looks
enough like Caroline Blakiston that you won't really notice/care she's not exactly the same actress from 1983.
But because the film ends literally
minutes before Star Wars starts, they think "it'll break immersion if we have a completely different actor playing one part and then when you watch on home video, it's some totally new old guy in the next movie."
I mean, that's basically it. There's really no way around that. They decided to resurrect Peter Cushing, and de-age Carrie Fisher, for
continuity's sake.
Which on the one hand, makes sense, but on the other, is a lot of work, a whole lot of it, with questionable returns, solely because you don't want the audience to take the two-and-a-half seconds to register that the fictional universe now has a new actor playing an old role.
(even though there's a new Mon Mothma right there, and we're about to get new Han Solo and Lando Calrissian next year, so...)