This is actually quite understandable. Although this event is said to be the Supreme of the Universe's ascension celebration, it's clear to perceptive people that this is the victory party for those who defeated the Primordial Honor. Sun Wukong simply wanted to take the opportunity to announce some matters at the celebration, so only the remaining majority of people outside these few Great Divine Nations aren't qualified to walk up the stone steps to the mountain; they can only take the side paths.