The understanding is that a decreased episode count has allowed the creators to up the budget and quality on each individual episode.
The final season will of course not be the end of the Game of Thrones universe though, with four spin-off ideas currently being developed with a view to greenlighting the best one.
Game of Thrones showrunners David Benioff and D.B. Weiss have long been saying they intend the final season to consist of just six episodes, and now it appears to be confirmed.
Such a shortened count will make the final batch of episodes almost like a mini-season and will likely be pretty action-heavy.