People make rational choices based on money and time. If people do not find off-peak hours reliable or convenient, they won’t use it. So demand is low because frequency is too low.

Other cities have fixed low-ridership by making public transit better. https://www.ctvnews.ca/ottawa/article/ridership-on-o-train-line-1-not-high-enough-to-justify-restoring-in-off-peak-headways-oc-transpo/