While we were in an agrarian state, and an early industrial state,
we a) were of very little use, b) had little to offer, and c) came at a
time when the Centauri were starting to fall back into isolationism just
a bit. The Narn had the misfortune to be strategically well located, had
many resources the Centauri wanted, and provided other advantages. One
doesn't just conquer worlds helter-skelter; it takes time, money, effort
and some blood to conquer worlds. You only choose those which offer you
enough to make the process worthwhile. That simple.

jms