Because Phlox running into xenophobic asshole in a bar who looks for a fight leads to an actual scenes. So does Trip and T'Pol infiltrating Terra Prime. Gee, it even leads to a storyline.
Rioting and protesting wasn't a scene. If memory serves the Andorian ambassador merely mentioned the riots. Why? Because shooting actual riots is expensive and because it puts the focus on the wrong people. The stories in Trek are virtually always (the VOY story with the reptilian aliens is the only exception which comes to mind) told through the eyes of the main characters.
But even if you wanted to show that xenophobia declines, how do you wanna do it? Showing xenophobes is easy, showing the absence of xenophobia, i.e. ordinary people simply not minding the presence of aliens, is hard.
It's like when Hamlets talks about the excesses in the court to make the audience feel that "something is rotten in the state of Denmark". Same when Polonius sends somebody after his son, it is only there to create the 'everybody spies on everybody' atmosphere of the second act. That the guy goes after Laertes, the pure plot, it absolutely irrelevant. The atmosphere matters, just like during the ending of TUC and Terra Prime.
In prose you can have a first-person narrator who tells about events in far off places. You can also do this via multiple first-person narrators. In TV/cinema you can also use a narrator, a choir (or Star Wars famous cards which serve the function of a choir) or tell the story through the eyes of several characters. This became popular during the 00s when many movies were of an episodic nature (and often the stories of several people "accidentally" crossed). But that's now how Trek works.