MargoChanning responded to a post about Cabaret also being a limited run but playing for years:
But when Cabaret was about to shift from Roundabout's LORT contract to the standard Broadway Production Contract (a substantial increase in weekly running costs) it had an advance of over a million dollars, so it made sense for Roundabout to continue on with the show in an open ended run. It was reported that "Assassins" has an advance of $90,000, so once all the subscribers have seen the show (which is very soon -- the reason grosses look so high is that Roundabout has something like 50,000 subscribers), weekly grosses will drop substantially and the show would start losing money. Single ticket sales have been extremely weak, so Roundabout, being a not-for-profit not willing to sustain huge losses, is doing the financially prudent thing and closing the show.
I have my books and my poetry to protect me...