Why is the Spanish Flu Pandemic the worst ever in world history?

It started during World War I. The close quarters and miserable conditions of trench warfare contributed to the spread of the disease. Soldiers who became ill were sent home from the front, which spread the disease further. When the war ended, they all went home, which spread the disease throughout Europe and to North America. It continued the following year, killing millions more.

It's not.

It's the worst influenza pandemic, but not the worst pandemic.
That would be any one of several outbreaks of Plague.

Because it killed an estimated 50m worldwide

It is not, the worst ever was Black Death.

It killed over thousands. Actually millions.