It is a popular dish to serve at Rosh Hashanah, Hanukkah and Passover! Lots of reasons - some tied to Ashkenazi traditions and the accessibility of cheaper cuts of meat. But in modern times, it's the tradition AND the convenience. Brisket can be made ahead so you don't have to do all the cooking on the day of an event or celebration. It's also easy to add vegetables to the dish to make it a one pan meal.