Airlines tend not to announce discontinuation of routes too far in advance, as it would effect sales of that route on dates prior to shut down. As long as they have a plane flying a route, they want the seats as full as possible.
Plus there is the aspect that decisions to shutdown a route are not made far in advance, airlines start a route and they do their best to make if profitable. the decision to shut down the route comes only after the airline has exhausted options to improve bookings on that route.
In many cases the announcement of a route shutdown is not made public immediately. Often elite flyers effected by the route shutdown are the first to hear, followed by high producing travel agents, then eventually the general news outlets and public.