GRRM makes an eloquent apology along with an observation that the show and the books have been diverging for years. 

As for explaining why it's taking so long, he matter of fairly states that sometimes writing takes longer than we want. 

Better to do it right, than on time.

That's what GRRM believes.

But in the meantime, his article prepares us for the fact that the series is now ahead of the books and likely to spoil some things that will happen in the books.

Because even though details of minor characters are different between the show and books, it seems like the story arcs of the major characters are very much the same.