How about this idea for how and why:

Kingdom was found at place X.
Original use of cup tied to place X.
Capital moved to Place Y due to _______(ummm I don't know.. I can't be too serious unless you have a period where the was no king due to not being able to reach place X).
Each year on the summer solstice(fit some specific date in here), the king must drink from the cup while at place X or lose his right to be king.


This scenario ALSO could lead to a future adventure where an army wants to invade and thus use an opportunity to steal the cup during the travel period to depose the king. Of one of several other scenarios. This also provides continuity and plot hooks between adventures(I would suggest any follow on adventure be somewhat different in nature though and quite a number of levels higher.) For example, instead of the cup being stolen, perhaps a doppleganger has replaced himself as king and the hero's have to intercept him before he can be anointed king. Of course, only a few people know he is not the king and the players have to figure out a way to subtly expose him prior to the big day. This would be quite a bit different from the original adventure but still have ties back and continuity.