Because the entire expedition didnt die! Only the final members of Scott's party were killed. After a year spent laying supply depots along the planned route South, several smaller side exploration missions, and preparing, the main effort began in September 1911. And involved multiple larger teams, some using motor sleds(which of course broke well before they got anywhere) others using dog teams and ponies(to be shot later for food) getting progressively smaller at intervals, after helping haul supplies the others would turn back, and prepare to meet the Pole team on their way back.

In the first week of January the final group was instead changed to Scott plus 4 men so 5 total, they reached the pole in mid January 1912, and began back North. On 17 February the first member of his party, Edgar Evans, died then at the base of the Beardmore Glacier, on 16 March Lawrence Oates voluntarily left their tent and died after suffering from severe frostbite, leaving the final 3 to trudge on. Scott's condition was worsening with frostbite, and their progress each day was slow, they finally died around 29 March some 11 miles from the next depot after several days in a severe blizzard and unable to leave their tent.

They were found there on November 12th by a party led by Edward Atkinson, an officer of the expedition and Royal Navy Sergeon who found himself the senior officer at their base camp over the Antarctic winter. He had led or organized several other relief attempts, but controversy still surrounds them today, such as who led them, the timing of them, if they were in accordance with Scott's intent or the ever changing plan with the Pole group. Though some setbacks also involved saving other members of the larger group sent back earlier like LT Evans who commanded the Motor Sled group and was near death from scurvy when his 3 man return group was rescued.

Scott and his groups effects were still in the tent when found and that is how the narrative from his POV was reconstructed.