Based on current guidelines from local public health departments, which recommend the cancellation of public gatherings involving 250 or more people, Cardinal Blase J. Cupich, archbishop of Chicago, mandated the suspension until further notice of all liturgical services effective Saturday evening with the regularly scheduled weekend services.

More Details in link: 
https://www.archchicago.org/en/statement/-/article/2020/03/13/statement-of-the-archdiocese-of-chicago-on-the-suspension-of-public-celebration-of-mass-the-temporary-closure-of-archdiocesan-schools-the-pastoral-cen