She died because she wanted. In anime she says that she is alive and wants to die to full became heroic spirit. She doesn't want to be betrayed, again, by her own people...
She wanted to die but she couldn't because she had Avalon within her. Than she completed contract with Holy Grail War. She competed with the master Emiya Kiritsugu. After war, when Kiritsugu found Shiro, he saw the poor boy was going to die, so he put Saber's Avalon within him. Saber returned to her real life it didn't pass second she was summoned again by Shiro. For her the time was frozen in her real life. She thought:...