Reading the Wikipedia entry for one of the showrunners of Game of Thrones, namely D. B. Weiss, one gets the impression of a struggling writer whose projects got turned down one after another. From the Wikipedia article (emphasis added):
Weiss and Benioff co-wrote a screenplay for a film titled The Headmaster, but it was never made. In 2003, they were hired to collaborate on a new script of Orson Scott Card's book Ender's Game, in consultation with the then-designated director Wolfgang Petersen. It was not used.
Weiss's 2003 debut novel, Lucky Wander Boy, is themed around video games. In 2006, Weiss said he had a second novel finished that "needs a second draft". That same year, Weiss completed a screenplay for a film adaptation of the video game series Halo, based on a script written by Alex Garland. However, director Neill Blomkamp declared the project dead in late 2007.
Weiss also worked on a script for a prequel to I Am Legend. However, in May 2011, director Francis Lawrence stated that he did not think the prequel was ever going to happen
Then, all of a sudden, he gets hired as one of the showrunners for big-budgeted Game of Thrones.
How did this happen? Is there any known explanation for this? How did a person with so few successes in film and television get such a massive job?