The Hightowers never truly disappeared. They just went quiet.
After the Dance, they lost their royal ambitions—but not their power. Oldtown, the Citadel, immense wealth, political connections, and Leyton Hightower still commanding an army kept their influence alive. By Game of Thrones, they were Tyrell bannermen, but the Hightower name still carried enormous weight. They didn’t fall off. They simply stopped being shown.