Consider what we've actually seen of Star Wars between the last two times Ahsoka was seen.
In "Rogue One", the story is focused on a small group of people and the battle at the end was a hastily thrown-together affair. Ahsoka could have been off doing Rebellion-related activities and didn't happen to be at the base when people scrambled.
In "Star Wars", we only see Rebellion personnel right before the attack on the Death Star, and that attack is going to be fighter-based. Even if she happened to be at the Yavin base, there's no particular reason for her to be a flying a fighter and again, she might not have been there in that brief period of time between the Falcon's arrival and the Death Star battle (implied to be only a few hours).
In the "Empire Strikes Back", she might not be on Hoth at the time of the attack. If not, again, that's the only time the main characters are among large numbers of Rebel troops or see what the Rebellion in general is doing. Otherwise, they're off on their own. We don't know what anyone else in the Rebellion might be doing.
In "Return of the Jedi", it's only right before the attack on the second Death Star that the heroes aren't off on their own little private mission to rescue Han that Ahsoka would have no reason to be party to. As for the attack itself, she has good reason not to be with Han's team trying to infiltrate because she knows there's a risk Vader and/or Palpatine might sense her, and that would blow the plan. She could be out fighting with the fleet, but if so she could be anywhere from in a fighter to on one of the capital ships, and if so, there's no particular reason to see her. She'd just be one of many others.
In essence, Ahsoka could be the hero of her own story fighting in the Rebellion, it's just that she doesn't happen to be in proximity to Luke and the others, since we're following their stories, in the very brief periods Luke and company are seen with other Rebels.