AskPhysics

Because they are different forces.

Gravity has an effect on the mass of an object by distorting space-time.

The particles in Electromagnetism are mass-less but travel through space time. Light is bent from its straight path in large gravity fields but not because gravity effects photons directly but because gravity warps space time.

Think about race car entering a banked turn. The car is the particle and the track is space-time. The car changes direction due to the curvature of the turn.