I'm thinking some reasons could be:
-parallel cables allow the cable to be tangential to the servo pivot point and the hinge axis at neutral position
-less complex geometry allows for easier adjustment/setup to remove any possible slack

crossing the lines theoretically should work OK, but for me it's easier to see exactly what's going on with the system visually. if something is slightly 'out of whack' I feel it would be easier to see with a parallel setup.

I'll have to think about this some more when school is done for the day, there may be other geometrical reasons (such as control surface sensitivity or error amplification due to geometry, etc.)