The same reason so many shows get cancelled after a few seasons: producers, networks, actors, etc. Just want to get paid and fans getting a complete series is not even a secondary concern. GRRM is a big name, and working on a project attached to him will bring in viewers and looks good on the resume regardless of how well the content performs.
Based on the trailer, did anyone think "In the lost lands" was going to be a blockbuster? But less than $7M in 20 days on a $55M budget? They basically said: "its GRRM, people will come see it". But those actors can add to their resume, "I was in a GRRM based film" and some (uninformed) producers and directors will mark that as a checkmark when deciding if they want to call them in for an audition!
I don't get the fuss, its not like GRRM spent years writing the DNA sequance for the wolves, at most he wrote a cheque to contribute... And some appearances here and there.
