Partly no doubt because of the sheer difficulty of separating the passengers into two groups based on where they've flown in from. Once they've landed and disembarked, all the passengers are mixed up together, so at the point of entry to any special security queue there'd need to be detailed checks of used boarding passes to make sure that they were (a) from the same day, and (b) matched the name on the holder's passport, which in turn would have to be matched to the person. In addition, there'd still have to be the usual checks relating to the next flight.