Projects in the film and television industry can stay in "active development" for close to a decade before they actually happen, depending on the project. You get money from this producer based on this actor's involvement, they seem good to go, you get others to sign up and then the producer pulls out and takes the actor with him. Now you gotta look for the money he took with him and an actor to replace the vacancy and in that time, the other actors you signed up sign on to other projects and when you finally get someone else they're booked and then the heads of the studio shift (which actually happened at Showtime between announcement and today's remarks) and the project has to go through the greenlight process all over again.
In short, this is business as usual when it comes to big budget productions like I'd assume a Halo TV show would be. It's gonna be a while before anything happens with this, if it doesn't ultimately get cancelled.