Plan A: Since it's called the Golden Bus
Ticket, perhaps they are not able to get on the King Bus unless they find the golden ticket first. And this ticket is of course hidden in Alien's super cool tie.
Or a Willy Wonka chocolate bar.
Plan B: The Golden Bus is discovered by Peter who is then threatened by the angry duo of Morsul and Death (who both want revenge on Philip), and is forced to run over his own father, thus becoming The Phantom.
Plan C: The King Bus is controlled by an evil pink kitten that wants to kill Alien for eating his BFF the Pheasant.
It's obvious which is most likely.