The fire doesn't stop at sunset. The helicopters do.
No night flying
Night is when humidity is up, wind is down and fires are easiest to stop — and when almost no helicopter is flying. Only a handful of agencies have a night-vision program; contractors stop at sunset.
Power lines kill pilots
Low, fast, in smoke, on terrain the pilot has never seen: wire strikes and terrain collisions are the leading causes of fatal aerial-firefighting accidents. The pilots lost are the most experienced ones.
Coordination by voice
'Drop on the big ponderosa left of the rock.' Targets are described by radio, sequencing between helicopters is called by hand, and nobody has the same picture. Drops land metres from where they were needed.