If Screed is in this game why not Thrawn?
Because Screed was part of Droids cartoon, and that is more canon than most parts of the Extended Universe at this time.
Actually, those cartoons like the rest of the EU novels were thrown out all at once. You'll remember that the canon reset only kept the Clone Wars and the movies. The rest, Rebels, new Marvel comics, new novels, has been added on since that time.
Now I suspect, if MSW is right like they usually are, that Thrawn will be back in the canon soon so maybe they are waiting for that to happen to unveil the new version of the character. I guess we'll just have to wait and see. Speaking as a fan of the old EU, who was glad to see Droids, the Vong, and other such bad ideas go, I'm hopeful they at least save or reimagine some of the better parts of those old stories, just like they brought the Interdictor back in the new novels and Rebels and just like they adopted a significant portion of the old Star Wars atlas into the Ep 7 visual guide.
Well, the rumour is that Thrawn will be Midway through Rebels Season 3, and they're just about ready to Wrap up Season 2 at the end of the month...