You kind of explained the motivation for this. Commercials. I don't know that I'd agree that "most of the tv watchers aren't watching it live," though. I'd guess the vast majority of them are watching it live. Maybe not on TV through a cable provider, but they're streaming it live in some way, shape, or form. And that has commercials. Which is how fox gets paid, and what they base their bids to MLB on. It's incredibly important for the business side of MLB to stagger their playoff games and move it to primetime if they can. But it is definitely not great for fans.
I think the thing they should've done was flip today's games. Put Texas/Baltimore in the afternoon slot so they know the result of that game earlier at least. But this is how it's always been, and always will be. The TV revenue is what drives almost every decision because it makes up the vast majority of the league revenue.