Because the very close miss in 2029 will change its orbit and differences beyond the precision of our current data will determine whether or not it hits in 2036. For more details check out my post #18 in this thread.
This is NOT the usual Chicken Little or the other usual reach for funding. If we do nothing we won't know whether we REALLY have a problem until too late. Relatively modest funding now or in the next few years will let us know whether to panic in time to panic effectively. If we're going to have asteroid rendezvous missions anyway, just pick this as your target and include placing a radio beacon on it as part of the mission. That would resolve the uncertainty. If OK then continue leisurely research on asteroid interdiction. If the long shot is going to hit than it trumps everything.
The Planetary Report article from last summer, outlining the problem and explaining why we may need a mission to it in the next decade to determine if it is really a problem in time to prevent a disaster, is finally online here.