LIGO collaboration member here: we focus on maximizing the cumulative number of detections over time, and it turns out that by turning the detector off for a year to upgrade it, we can increase the sensitivity so much that a year after that we will have more total detections than if we had left it running continuous for two years. So you also have to think about all the distant/faint signals we would never be able to detect because we kept listening at too low a sensitivity...

(We do make sure that we have a dramatically less sensitive sister detector in Germany, called GEO, listening whenever we're not so that we'll see something really close and loud, like a galactic supernova, even when LIGO is offline.)