Why Overcurrent protection must interrupt the current, so the fuse or breaker goes in series with the conductor carrying the supply voltage, which is the hot (ungrounded) side. Opening only the hot conductor guarantees that when the device trips, the equipment is disconnected from the 120V source and nothing downstream stays energized. The National Electrical Code prohibits putting an overcurrent device in the grounded neutral, because a blown neutral fuse would leave the gear still connected to the hot line while appearing dead.
Watch out Fusing both hot and neutral sounds like extra safety but is actually a code violation and a hazard: if only the neutral fuse opens, the chassis can remain at line potential even though the equipment is off.
Fuses go in series (they must break the current) and only in the hot wire. Never fuse the neutral.