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, oleh 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 detik she was summoned again oleh Shiro. For her the time was Frozen in her real life. She thought: "I can finally die piecefully", but Shiro summoned her. Than he gave back to Saber Avalon so she can beat Gilgamesh. When she destroyed the Holy Grail she returned to her painful death and in her way back her wish was granted, she could die... So she asks her loyal servant, Bedivere, to take her sword and throw it in lake, the same lake that Excalibur (depicting Arthur receiving his sword from the Lady of the Lake) was diberikan to her... And then when Excalibur was separated from Avalone she died. :*( :*( :*( :*( :*( :*( :*( :*(


Here is some lebih explanation... XD


Her full name is Arturia Pendragon, and her creation as a character was inspired oleh the legends of King Arthur. Arturia is the daughter of British King Uther Pendragon and Igraine, a former Duchess of Cornwall. At her nativity, Uther realizes his subjects will never accept a female as a legitimate ruler, and decides to not publicly announce Arturia's birth atau gender. She is entrusted oleh Merlin to a loyal knight, Sir Ector, who raises her as a surrogate son. When Arturia is fifteen, King Uther dies, and with no known eligible heir to the throne, Britain enters a period of turmoil following the growing threat of invasion oleh the Saxons. Merlin soon approaches her, explaining that the British people will recognize her as a destined ruler if she withdraws Caliburn, a ceremonial sword embedded in a large slab of stone. However, pulling this sword is symbolic of accepting the hardships of a monarch, and Arturia will be responsible for preserving the welfare of her people. Without hesitation, and despite her gender, she draws Caliburn and shoulders Britain's mantle of leadership.

Arturia rules Britain from her stronghold in Camelot, and earns the reputation of a just, yet distant king. Under the guidance of Merlin and with the aid of her Knights of the Round Table, she guides Britain into an era of prosperity and tranquillity. Caliburn is destroyed, but Arturia soon acquires her holy sword, Excalibur, and Avalon, Excalibur's blessed sheath, from Vivian. While Avalon is in her possession, Arturia never ages and is immortal in battle.

Throughout her reign, Arturia is plagued oleh feelings of guilt and inferiority; she sacrifices her emotions for the good of Britain, yet many of her subjects become critical of her lack of humanity. Excalibur's sarung is stolen while she repels an assault along her country's borders; when Arturia returns inland, she discovers Britain is being torn asunder oleh civil unrest. Despite her valiant efforts to placate the dissent, Arturia is mortally wounded oleh a traitorous knight, a homunculus born of her blood named Mordred, during the Battle of Camlann. Her dying body is escorted to a holy isle oleh morgan le Fay and Sir Bedivere. Arturia orders a grieving Bedivere to dispose of Excalibur oleh throwing it back to Vivian; in her absence, she reflects on her personal failures, regretting her life as king. Before her last breath, she appeals to the world; in exchange for services as a Heroic Spirit, she asks to be diberikan an opportunity to relive her life, where someone lebih suitable and effective would lead Britain in her stead.