If you need a reason... Uhhhmmm.. The connections aren't perfect and degrade material sent through them. One hop is ok, but things get real fishy with 2. That's also the reason you can't send people.
Plus the paper work, the merchants need sleep, travel that fast means bad things for bodies, has to match it with perfect alignment of the sun & the gods get a little annoyed
EDIT: Anyone with Regional Government: is it worth it? And does it collect from only your own planets, or other people's too? I was thinking about popping one on my home planet to get a bit more cash in to pay for the food it has to buy but running 20% tax rate and being right next to Kai makes me think twice about it.
