That is a good question but you must remember that there is an abundance of mail at this time so that is the cause of the delay. They always tell people to mail early and shows the cut-off date if you want it arrived before Xmas. It is possible that it has left already, on route here or here already. It might soon get here and plus the postmen will be working this weekend as it is the last before Xmas.
Well I dotn know what to tell you. Did you try calling the post office to see how they regulate the mail? Maybe you should try that and see what the feedback is.Hope it works out!?