For me, the difference seemed to come to to the bullpens. Both teams were evenly matched in many ways but the Astros had the superior bullpen and that made the difference. I could be wrong, I haven't looked at the bullpen data, but it seems like every time the Red Sox went to the bullpen, runs were given up. In contrast, the Astros bullpen was much more effective. In game 4, for example, it looked like the Red Sox might cruise to an easy win when they started to beat up on Zack Grienke, but Baker went to the bullpen early and shut the Red Sox down.
Outside of Whitlock, can you name one Red Sox reliever that you trust? In retrospect, Barnes' implosion ended up being a big deal, as that forced Whitlock into a late inning role and weakened the set-up relievers. Ottavino had his typical late season struggles. Robles had his moments but he isn't a guy you are going to trust in a big spot. The Red Sox have a big challenge next year: they must rebuild the bullpen. And if they plan to move Whitlock to the rotation, that is one less quality arm for the bullpen.