Why was this attack such a shocking ack

The Birmingham pub bombings were carried out on 21 November 1974, when bombs exploded in two public houses in Birmingham, England, killing 21 people and injuring 182 others.

The Provisional Irish Republican Army never officially admitted responsibility for the Birmingham pub bombings, although a former senior officer of the organisation confessed to their involvement in 2014.[4] In 2017, one of the alleged perpetrators, Michael Hayes, also claimed that the intention of the bombings had not been to harm civilians, and that their deaths had been caused by an unintentional delay in delivering an advance telephone warning to security services.

Explanation:

the answer should be monarchy