The Mahabharata says Bhishma gave up the Kuru throne and vowed never to marry so his father could wed, and the line ran out of heirs

The Adi Parva says King Shantanu wanted to marry Satyavati, a fisherman's daughter. Her father would agree only if her son became king. He also feared that the crown prince Devavrata's own children would later claim the throne. Devavrata promised that her son would be king and gave up his right. Then he vowed lifelong celibacy so he would have no sons to challenge it. The text says the gods called him Bhishma, "the terrible," for the vow. His father granted him the power to choose the hour of his death. Satyavati's two sons died without heirs. She asked Bhishma to marry the widows of the younger son or father children by them. He refused, citing his vow. The sage Vyasa fathered children by the widows instead: the blind Dhritarashtra and Pandu, whose sons fought the war at Kurukshetra (story 17). Bhishma commanded the Kaurava army for the first ten days. He fell on the tenth day and lay on a bed of arrows for 58 days, advising both sides, and chose to die after the winter solstice.