From what Ive heard, the reason they cut the episode count down was apparently practicality. Season 6's production was long and massive. Multiple units working qorld wide for nearly a year. It was reaching the limits of what was apparently reasonable and sane for the crew to pull off. Espically for a television show. They couldnt increase the scale and scope of ten episodes any further without the production schedule becoming unwieldly and too exhaustive. So it was either keep the 10 episode number and not increase the scale of any of the episodes beyond what we saw last season. Or cut the episode number down to 7 and do those 7 episodes in the same production time it took them to do 10 episodes of a previous season.
I think that speaks volumes to the scale of the work the crew is doing. They cut back to 7 episodes but didnt cut down the amount of work they were doing or afaik the budget of each episode. When this season is over we will have seen the full grown dragons more than in any other season, we will have seen one large true battle involving Drogon, a fight beyond the Wall involving the army of the dead, we will have seen the white walker army more than in any other season, we will have seen and spent more time in a number of unique and new locations (Casterly Rock, new areas of Dragon Stone, East watch, the Citadel, parts of the westeros grass lands etc)...its a scale and scope that exceeds previous seasons in a number of ways. If they had kept the season at 10 episodes some of this might have been cut simply because they didnt have the time to get it done in a year.
They could have kept it at 10 episodes and extended the production time by months and just bumped the season release back to 2018 but I doubt HBO or D&D really felt like doing that. So here we are with 7 episodes instead of 10.