Why? Because it was made for U.S. Television.
Since this is not a TV show we either go by the more likely percentage of humans in Starfleet, based on the fact that there are over 150 planets in the Federation that percentage should be far less than 50%.
Or we need a "filter" that removes all those inapropriate non-anglophone human names and then removes all those non-human chars.
Then we can rename the "United Federation of Planets" to the "Federation Reich of Anglophones".
however I do agree that the majority of the npcs/crew on most ships should be human to reflect the shows, granted places like DS9 the bajorans should be more prominent along with other alien species...this also reflects the shows.