Aerial firefighting is expensive. Tankers cost upwards of $6,000 per hour to operate. In 2018 the U.S. Forest Service spent over $500m on air tankers, helicopters and other firefighting aircraft.
It is also extremely dangerous. To be effective, pilots must fly large aircraft no higher than 200 feet above treetops. Thirty-seven airmen have died in aerial firefighting accidents in the last decade.
The goal of aerial firefighting is not to put out the fire directly, but rather to support the ground crew. A large plane dumps red retardant around flames, not on them, in order to build a fire line. A helicopter dumps water directly onto a wildfire not to put it out, but to reduce the heat to allow the ground crews to encircle the area with firebreaks. In this, they are essential.
However, manned aircraft operations are expensive, and wildfires are becoming more prevalent. They happen each year in different hemispheres and rarely at the same time. Unmanned aerial assets can get the airmen and women out of the loop, and make the aircraft that fight them transportable.