Because even a fraction of their original viewership is still really good ratings. And they know a good chunk will watch every bit of new stuff that comes out.

What I can't figure out is how there's an audience for Walking Dead games outside of the Telltale series. It seems like the people still watching are middle aged women who got hooked by Daryl and drama. Most of the gamer types I know who used to watch it, fell off during the Negan season. Plus every new game has had horrible reception.