Cargo securing on road vehicles

Why doesn't the cargo slide immediately upon braking?

This is the work of frictional force between the cargo and the contact surface.
Frictional force is the force working against the shifting of a body on the surface under it (i.e. slipping). It depends heavily on the type of surfaces in contact.
A dirty cargo area reduces the friction dramatically and thus promotes the goods to slip.
Be aware of the nature of the load platform e.g. wooden platform, checker plate platform, smooth or rough surfaces.

Remember: The rougher the surface, the higher the friction.