The MED flights, AFAIK are strictly inbound during Hajj season. Their long term plan is to shift all Hajj flights originating in the west to arrive at MED instead of JED. This will ease some of the crush on the road infrastructure between JED and Makkah just before Hajj.

There is also a religious advantage in going to Medina first. If one arrives at Jeddah, one is required to be in Ihram and to perform an Umra as soon as one arrives in Makkah. If one travels to Medina, there is apparently no need to be in the Ihram. I do not know what the reason for this is (yet).