The production closed during the 1968 Actors Equity strike that affected Broadway shows. The strike probably hastened the closing at least by a bit. Prior to the closing, there had been no announcement of an imminent closing nor any advertisements that the show was in its final weeks, although the fact that seats were readily available at all prices had been mentioned in ads.
Hiram Sherman won a Tony even though he'd left the show before the Tony awards, possibly even before the nominations were announced. He was definitely gone by the week after the nominations were announced.