Because it was one of the biggest ''Massacres'' that ever happened. Also if youve looked at the photo the red coats used guns and knives while the pariots used only axes, picth forks and anything they can get their hands on. Especially since, more than half of the people died on the patriot side than the red coats.
To gain support for the revolutionaries. The story was exaggerated, but the American people bought it. It was used as a recruitment/propaganda campaign to turn as many people against the British occupation as possible.
