I imagine one reason could be that directors or producers or whoever the hell is the one behind it feels that a big scale war might be... Too much? Too much action, too much money being spent on CGI for the ships/effects/whatever, too much... I don't know. When I say too much action, I mean that not everyone is a big fan of large scale fight scenes going on for too long, and I can see why. It can be a bit boring after a while if all you see if people on screen shooting at each other or bombing each other over and over and over again, especially when there are a lot of nameless soldiers running around without any names or personalities. Viewers don't take stock in them, and therefore, they don't care about who lives or who dies, which then translates to them waiting for something meaningful to happen. Sure, they could enjoy the special effects being utilized or they could enjoy some big scene or two, but when it's a massive war going on and there are no characters of importance, it's not as exciting.
Now, I know, there are ways to fix this. Filmmakers could have characters on both sides of the war that are established early on, with personalities and back stories and enough to make viewers care about them, or at the very least, interested in seeing what they do. Then, it's simply a matter of shooting a large scale battle in a way where different viewers can actively enjoy it. Like the Lord of the Rings fights, for example. Those are big, with numerous characters duking it out, and while there are scenes where nameless characters fight and die, they're interspersed between fights or scenes with named characters people care about. I feel like, for a sci fi war type movie, the same thing needs to be done and it would work out well. Have characters that the people care about fighting for their causes, have them struggle, have them get injured. Make it look like there's a chance the good guys can die, and maybe even kill off a hero or two during the battle in order to show that it means something. The problem with Star Wars when it comes to their big battles is that... Well, nobody of consequence really dies or takes enough damage to be concerned. Yes, we have moments where characters like Obi-Wan and Han died, but in the heat of the moment, in those big epic space battles, we don't know or care about the other characters getting blown up.
That's just my opinion, though.
