Mosconi famously didn't like to play pool more than required to satisfy his sponsors, and his exhibition schedule was grueling, so I suspect he happily ended his runs as soon as possible. Most of the public wanted to see his trick shots anyway. Why he stayed late that night for his big run I don't know. I can't remember if he addressed it in Willie's Game.
Greenleaf also had a demanding exhibition schedule that would have limited his ability to grind for high runs. But as early as 1919 the press (and Brunswick) was promoting the high runs of top players. I also wonder if the poor equipment (uneven slate, clay balls, ratty cloth) made luck more of a factor in the high runs of the old greats.