Folks that watch the show aren't necessarily tied to the books thoI feel like if Gandalf, Radagast, and Saruman aren't in this time zone then that'd be a legit complaint if the showrunners included them. Wonder if it's some magic user before their time. That'd be nice to include someone new that was before them
It makes more sense to make it a character today's audience would know. And Radagast to me isn't strong enough of a character