Difference in flow velocity.
The simplest explanation for flow void: Blood in carotids have high velocity. So in this slice, other structures are excited by the magnetic field but the blood is NEW to the slice since it comes from the lower slice that was not excited ((we use slice selective gradients in MRI) ). As a result, you see it as a signal void (flow void). Stationary structures have signals because there are excited by the magnetic field in the slice selective gradient. The flow in Jugular vein is slow enough to behave like stationary tissue and not blood in carotids. If you don't see normal flow voids in carotids, you have to think about some vascular pathology like obstruction.
If you are interested in this topic, you can also read about phase contrast MRA/MRV and TOF MRA. TOF is somehow the opposite concept.
