From my own personal experience (
Steallaris, ahoy!), when you are expanding your empire early on and you have very limited resources to build a colony ship, you can very well happen to have surveyed the regions around your empire and find one side full of perfectly colonisable worlds, rich in minerals and energy and science, and the other side off of your empire to be barely inhabitable rocks without any worth while resource concentrations.
From a simple economic standpoint, it becomes painfuly obvious which area you should pay more attention too.
Of course, in an ideal world where you have infinite resources, the right thing to do would be explore
all ze things!!1!, but when you have a real, finite economy and resources, you simply have to prioritize:
"What is more cost-effective?"
I don't think it is far-fetched that the Loroi haven't explored that region of space before - there was simply no need to. Why expand into a scarcely inhabitable and rather poor area of space, far away from civilization and estabilished logistic lines (thereby making the colonization or exploitation of two exactly equal planets cost more in the uncharted space) when you have perfectly colonizable and exploitable planets right inside your territory?
TL;DR - When I play Space Emperor, this thing sometimes happens to me.
