Actors’ contracts is definitely a consideration, though this is a show where no character appears in every episode.

Also, 10 episodes would mean a whole other month that people would subscribe to HBO than if it were just 6.

The idea that the show runners are floating that the show had to be exactly 73 hours, no more, no less, is ridiculous. Somethings in the first few seasons took forever (how long did it take Bran to get where he was going, or Daenerys?) whereas in last weeks episode

Tryion goes from the battlefield, to Dragonstone, to King’s Landing, and then back to Dragonstone, while Gendry appears for the first time in four seasons in King’s Landing and then is beyond the Wall before the hour is up (and those two examples are just the tip of the iceberg).