When Van gets captured at the start of ep5 and Allen tells Merle and Hitomi they shouldn't worry about Van's life, at least, he says, "They say that only the King of Fanelia can operate Escaflowne."
So:
1. There are widespread knowledge and rumors of Escaflowne as a legendary guymelef surpassing normal guymelefs, and that it belongs to the Fanelia royal house, passed from king to king.
2. It seems almost logical to Allen that Zaibach has waged war on Fanelia to capture such a commodity.
3. There must be a bond between the royal blood of Fanelia and Escaflowne beyond merely possessing the guymelef. In the fight with Allen and Dilandu, Van seems to take personal hits from actions on Esca--this is even before strengthening his blood pact.