Simple. Game of Thrones is one of the most popular TV series of all time. Next year HBO will air the remaining 6 episodes and then that's it. It's done.
Amazon sees a built-in audience of GoT fans.
I hope they succeed. I love GoT and will greatly miss it. If Amazon can bring the same high quality to their program, it's a win-win for everyone.
Joe Abercrombie s First Law series would make for a good if not grim series if done right.