Shooting later in the year makes sense because even Spain and Croatia (locations used for King’s Landing and other places in the South) have a change in seasonal weather, more grey skies and longer nights, so that helps them sell the arrival of winter.
But I think there are two reasons why they reduced the episode count; one is that it takes the pressure off the production a bit. Each episode can shoot for a bit longer, post for a bit longer, run a bit longer!
And secondly the show is great for advertising HBO, they get it on air for an extra year. Less episodes but still, an extra year of hype and publicity.