Why did the barons force King John to sign the Magna Carta?

He forced him to sign to be able to get certain rights.

King John needed the support of the barons in ongoing hostilities with the French and the Pope. The Magna Carta (1215) was important because it subjected the monarch to the rule of law and documented the freedoms held by free men. These rights included guarantees against false imprisonment, land seizure, right to lawful judgement by peers and no delay or denial of justice.