Why does Alcestis die in place of her husband Admetus?

In the play Alcestis by the Greek dramatist Euripides, the title character dies in place of her husband, Admetus, due to a divine arrangement involving the god Apollo.21

When Apollo learned that Admetus was fated to die young, he persuaded the Fates—the goddesses who govern human destiny—to grant the king an extension of his life. The Fates agreed to this on the strict condition that someone else must be found to die in Admetus's stead. As a devoted and loyal wife, Alcestis voluntarily consented to sacrifice her own life to fulfill this requirement and save her husband.213

Ultimately, the hero Heracles intervenes by wrestling with Death at Alcestis's grave, successfully rescuing her and restoring her to her husband.213