The gate was set up in what was believed to be a reasonably fertile
and useful sector of space; the explorer craft can't take too long in any
one area. They find likely areas for exploitation, take a fast look
around, drop a gate, and move on. This allows other ships to come in and
scope out the place in detail. That's when they ran into whatever's
walking around Sigma 957. (And it's not there all the time, only from
time to time.)

jms