Here's why:
* The DA is disbanded. After the Ministry finds out about the DA, it's shut down and Harry, Ron, and Hermione are forced to flee.
* Most DA members are at home. Since the DA is disbanded, the students are all at home, with their families, over the summer.
* Some DA members stay at Hogwarts. Some of the older students, like Luna Lovegood and Neville Longbottom, choose to stay at Hogwarts, but they are under strict control and are not allowed to form any gatherings or groups.
So, while Umbridge takes over Hogwarts, the DA is not present at the school. They are disbanded and spread out across the wizarding world, waiting to reunite for the next school year (and ultimately, the battle against Voldemort).