Because, like your quote implies, it puts humanity above every other established race in the galaxy. Gene Roddenberry once said "If we aren't writing about humans, what's the point?" and that's a quote I detest because, now I know this is going to sound silly, it's flat out arrogance and prejudice towards the other races in the show.
The Star Trek franchise has had many major reoccurring characters who were not human. Some would be Vulcan, Klingon, Bajoran, Trill, Cardassian, Changling, whatever Neelix was ect. To say that we should focus completely on humans and Earth when we have so many other races with stories to tell is in and of itself a prejudice. We are putting us above every other race for no other reason outside of the fact that they are not human. Star Trek may not always have been the super intellectual show many claim it is, but at least a big chunk of it was open minded about the universe in which these characters inhabit.