Simply put—books allow for a lot of things. Mostly getting into a character's head and reading their thoughts. The real factor is that the media you listed has plenty of room to expand upon the era of the stories over time as opposed to just several hours of content.

We see this done spectacularly in TCW once it really gets the ball rolling. I'm almost done with it and it simply does a good job of expanding the prequels and arguably showing the fates of characters that are absent by the time of the OT. The galaxy is bigger than the Clone Wars because they move around with characters in a serial format whilst inching towards the end goal of ROTS.

The movies don't have that type of time to show all that and have to be condensed and concise. They aren't bad but I certainly prefer the show format. Moving forward I think there is a lot of potential if they want to worldbuild in an era, followed by movies that are the culmination of that or a brief blurb if relevant.