With 'Clone Wars' the narration worked both as a way to allude the text crawls from the movies and it enhanced the feel of the wartime setting by evoking old WWII newsreels (which directly ties it to the old Saturday matinee serials from which all this was inspired.) It also have a very practical function in tying the narrative together since the story didn't always focus on one particular group of characters and would tell all sorts of disparate stories.
Rebels has a much tighter focus both in it's scope and it's cast of characters. Also not being quite so serialised there's no need to catch the audiences up very time. As mentioned above, all this comes from the old Saturday matinee serials that were little 5-10 min episodes that always ended on a cliffhanger and were probably often shown out of order. None of that is a concern with Rebels so they just play the fanfare and get on with the adventure.
Honestly I'm not sure which way they'll go with 'Rogue One', though my gut says they'll likely keep the classic fanfare & text crawl intact. If nothing else it helps to serve as a nice familiar way to ease in audiences before changing things up, but also help place the story in time. Since this takes place right before ANH, the Han Solo movie *probably* happens about 10 years before that and if the Kenobi movie happens it could take place even earlier or between the two. Point is, these movies are going to jump around in time a bit and not always feature familiar characters or settings (even more so if they ever to something like a KotOR movie.)
The crawl is a useful tool to get the basic exposition out of the way without the audience actually feeling like they're getting an exposition dump and instead accept it as a convention of SW movies.
