I would say that 4X games traditionally don't have a campaign because each individual map takes so long to complete....
Who said 4X Games don't have campaign in the first place?
Homeworld 1->Awesome Campaign
Cataclysm -> Even Better story
Homeworld2 -> Pretty good campaign
Haegemonia -> Thrilling Campaign
Empire at war -> Ok, standars Lucas Arts-storys
Imperium Galactica 2 -> Even three different campaigns
Nexus:The Juppiter Incident-> So much campaign, there wasn't even enough budget left for a real multiplayer

(Not a real X4 but anyway)
Even Master of Orion 2 has some sort of higher goal with defeating the Andorrians(or however they where called). Furthermore there where different ways to win. Not only to crush as much alien scum as possible.(And when you're done continue with harassing former allies)
So on first thought i know more space-RTS with story than without one...