I honestly think they're too afraid to put all six movies together because of how polarized the fanbase is. There are a lot of people who only like the original trilogy and don't want to waste money on three other movies they don't want to watch. And then there are those people who only like Star Wars who wouldn't want to buy five other movies they didn't like. So I think continuing to separate them into trilogies is a safe marketing move on their part, despite what George would want us to believe about how they're supposed to be a hexology instead of two trilogies.