Asymmetric blades are more efficient than symmetric blades. But they cost more to make.
Symm blades have the advantage that the centre of pressure (and the pitch change moments) doesn't move much with changes of angle of attack, so the blade grips can be less strong, lighter, and therefore cheaper.
Asymm blades are now easier to make with carbon fibre etc, and the grips can be made stronger and lighter than in the 60s and 70s.
So, you should find that modern machines have asymm blades, older machines and those made to a price don't.