Which is something I don't really get seeing as what people were seeing was a hologram of Seldon activated at certain times via his predictions. They could use the hologram idea and still keep using the same actor. That's what confuses me. The only reason I could think of for going that way would be if they plan on doing two-way interaction with him.
Seldon isn't the only character in the story. They presumably want to keep multiple actors around across a multigenerational saga. Including Lee Pace, who's apparently playing the emperor clones (alongside older and younger actors as his predecessors and successors in the dynasty).
Let's face it, the books could not possibly be adapted with any kind of fidelity into a workable TV series. So there's no point even trying to apply the standard of fidelity here. All they can do is take the general concepts and characters from the book and piece them together in a new way, telling a new story of their own -- like how Marvel movies build new stories by cherrypicking assorted characters and storylines from the comics and mixing them together in new ways.