Mortis is an anti-aircraft sort of Dread, having special abilities to shoot deepstrike/flying stuff. The Helical Targeting Array that grants Skyfire and Interceptor also comes with some pretty stiff penalties if you think about it. Have to declare you are using it on your turn, you have to remain stationary, and if something doesn't deepstrike or fly, you kinda waste it.
The regular Contemptor does not have the the Helical Targeting Array, but it does have way, way more options for weapons than the Mortis. Volkite is more of an "anti-personal" deathray beam weapon then vehicle killer. The Contemptor has a
WS 5
BS 5, while the Mortis is
WS 4
BS 5. You are paying the extra points for that extra
WS and lack of Helical Targeting Array. An argument could also be made that you are paying extra points for the list of options you could take. All other things being equal,
GW/
FW seems make things with a larger optional wargear list more expensive. You pay more points for a long options list that you have to pay points for...