At that time Italians were not widely considered white and it was acceptable to see them as prime suspects of crimes against corporate property and terrorism. Also, everyone expected the trial and subsequent execution to go down smoothly.

https://www.smithsonianmag.com/history/sacco-and-vanzettis-trial-century-exposed-injustice-1920s-america-180977843/

https://andscape.com/features/white-immigrants-werent-always-considered-white-and-acceptable/