My question is why didn't the 20 amp breaker trip before the 30 amp fuse blew?
Could be one, or more, of the following:
[*] Fuse was Non-Time Delay type (Fast Acting),
[*] Terminations at the Fuse were generating heat, which assisted in a more rapid melting of the Fuse's element,
[*] Circuit Breaker's contacts are welded closed, from continuous reclosing on a faulted circuit,
[*] Circuit Breaker is of the infamous "FPE Trip-Free", or "No-Blow Zinsco" designs,
[*] Motor Circuit is maybe not fed "Entirely" through the 20 Amp Breaker,
[*] Non-Selective coordination between the Feeder's OCPD and the Panelboard's OCPD.
Sounds more likely that the Fuse was just a Fast Acting type.
At least the E base Fuse did not have a"Capacity Extending Device"
or "Trip Eliminator" (AKA "Coin behind the Fuse")