The main reason is traction. The front wheels of a forklift generally provide the power (acceleration) as well as the primary braking. Driving forward down a ramp (or backward up a ramp) keeps the rear counterweight higher than the body of the forklift, allowing gravity to maximize the traction of the front tires. If the vehicle was driving up a ramp without a load, the rear counterweight would be lower than the truck body. This could reduce the traction of the front tires, particularly if the operator is accelerating up the ramp.
Keeping the forks pointed downslope by driving backward up the ramp should avoid this risk. When the operator is carrying a load, of course, the load should remain upslope (whether going up or down the ramp) to minimize the risk of the load sliding off the forks.