To answer OP, its because its bad game design and FD doesn't think about wasting players time. Hopefully one day the make it so you can collect mission rewards, pay/collect bounties, and have ships delivered in/to any system or station.
I think there is some lore reason.
AFAIK
Jump drives in ships bend the laws of space time like black holes. You travel what feels like hours but to everyone else its a second.
As such communication, even travelling at the speed of light is going to arrive hours after you sent it. I mean look at the delay on Apollo mission communications it was in the realm of 30seconds I think just to the moon...
Its the other way around (at least in real world physics, not sure about game lore) as you travel faster and closer to the speed of light, time slows down for you. Don't know if that would apply if you are traveling at 100 to 1000 times the speed of light. So if you traveled at .99c in super cruise to get to a space station, what seems like a couple of minutes to you could be more like days (or more) to people waiting for you.
But... its a game and I think that the frame shift drives are basically just break relativistic physics so that players don't have to think about it

i think in the lore the speed of light has been broken by technology so its irrelevant in travel times and communications lag.
as to the intel delivery missions I think its assumed that they are either secret or maybe they are like huge files and the stations use Time Warner galnet and its actually just faster to have some one get in a space ship to deliver it
