I believe George R. R. Martin uses wordstar to write his books. I still hold a little hope that he will finish A Song of Ice and Fire series.
Maybe he hasn't finished it because he can't run WordStar anymore.
I think he is busy making sure AI doesn't finish it first. Can't have AIs trample in his fantasy land.
I have picked up from random interviews and other things the belief that GRRM finished the series but is considering publishing it posthumously to just not have to deal with fans picking it apart, especially because of reactions to the ending of the HBO Show.