The reason is something the general public would not easily comprehend.
But, if I could possibly provide some input on that situation, there's a fair bit of open runs every day that don't have operators to cover them. Seeing how that was for Sunday, the B37 isn't a very frequent line and there aren't many stand-alone runs for it on weekends, most B37 trips are interlined with either the B63 or B70 instead.