Screwdriver blades are made from one of two types of material; Chrome Vanadium or Chrome Molybdenum.
Why two different materials?
As you can imagine smaller screwdrivers can have a lot of torque applied to them to undo screws, in fact almost as much effort can be put through them as with a larger screwdriver.
Therefore, smaller diameter screwdrivers must have an exceptional resistance to being twisted. Chrome Molybdenum steel fits the bill nicely.
The drawback? It's more expensive than Chrome Vanadium.
As the screwdriver shaft gets bigger and increases in diameter it doesn't make sense to use a more expensive material when Chrome Vanadium is perfectly suited to deliver the necessary performance.