Also keep in mind that they likely have to evaluate their own financial stuff for the previous year before committing to the dining plan for the next year. As such, they can't really do it any earlier, as it's irresponsible for them to sign back up if they'd end up losing out in the deal.
It is what it is, but know that in the history of the dining plan, only 1 restaurant has ever dropped participation. This was Fulton's back in 2005 or 2006 (the year after the first year of the plan). For 2012, they came back on board and brought several other new places with them. So, while it's certainly not guaranteed, the track record of restaurants signing back up is pretty strong.