Fans Love Fantasy
One crucial point is that there has never been a better time for the fantasy genre. Fans love shows like Stranger Things and The Witcher. Studios are constantly trying to find the next Game of Thrones but very few stick to the pace (via CNET).
But HBO has an advantage because they are dealing with the real Game of Thrones universe. They believe that fans love the world and the genre enough to watch the new series. It’s a calculated and expensive risk but one that they believe will pay off.