Why did they name the title of the story the crucible?

It's a play, not a story--and a crucible is a piece of lab equipment that can heat things to really high temperatures. On the most basic level, witches were burned to death, so it relates. On a less basic level, crucibles create intense, hot situations--which is what boils up over Salem within the play. Various characters are trapped in dangerous situations and can't escape--just like the lab environment. Draw your own conclusions from there.