Because he was the first reigning monarch to have faced a public trial and execution.
The trial of Charles I was one of the most momentous events ever to have taken place in Westminster Hall. Kings have been deposed and murdered, but never before had one been tried and condemned to death whilst still King.
Following the end of the Civil War Charles I was brought to trial in Westminster Hall on 20 January 1649. The Serjeant at Arms rode into the Hall carrying the mace and accompanied by six trumpeters on horseback. The King's trial was proclaimed to the sound of trumpets and drums, at the south end of the Hall.
Bringing the King through a large crowd at the north was too great a risk; on the other hand, it was important that the trial be held in public. The court was divided from the public by a wood partition from wall to wall, backed by railings, and guards were stationed on the leads.
The King appeared before his judges four times, charged with tyranny and treason. The exchanges always took a similar form with the King challenging the court's authority and its right to try him.
Eleven years later, after the restoration of the monarchy (under Charles II), many of the surviving regicides were tried in the Hall, and nine were condemned and executed.
The bodies of the key men who ordered the execution of Charles I - Oliver Cromwell, John Bradshaw and Henry Ireton - were exhumed and their heads stuck on poles on one of the Hall's towers. Cromwell's remained there for more than 20 years.
Know more about the trial of Charles I at:
HistoryToday.com
Parliament.uk
Constitution.org