Probably cause they need so squeeze a whole storyline as well as battles into a 20-40 minute period lol

It's a time of war. Duels are less important in so many cases because they're so common. They'd get repetitive and throw off the episode pacing if they were as theatric as they are in the films.

Also, they're difficult as hell to animate, hence why the Ahsoka/Maul duel was mo-capped.