In a word, "JEALOUSY"
People want to find the weak-point so they can pull others down and be able to literally "look down" at them. It is in most instances, the human ego that is threatened by a person that seems to fail at one thing though they've succeeded and accomplished numerous great things in the same time frame. Look at how people twist things like when certain celebrities die "from drugs" . . . many times it's not drug abuse or addiction that results in their death but the simple fact that they took a sleeping pill or, in the case of Bruce Lee, an aspirin.
We veil our Jealousy under the guise of Patriotism as well as what is technically "Racism" and of course Religious Intolerance and Political Self-Righteousness. Then we have all that paper in our lives such as college degrees and from which University the degree actually came from. . . hell, you might have done two or three years study at one campus and won't get credit at the next because of petty B.S. between the schools and curriculum nuances. Yet, is is still a means by which we belittle one person or group so as to elevate ourselves.
Yes, it is "Human Nature" but philosophically speaking, it is one of the greatest "sins" or transgressions within the make-up of "man". Our challenge, if we are to listen to the old mystics, sages, prophets & gurus, is to overcome that lesser nature and discipline our ego as we would a child; explaining to that child that it can't have everything its way . . . essentially slapping out own hand when it comes to expectation and opinions that are self-aggrandizing and "slanted" . . . which isn't always tied to a single person but groups.
Until the human race as a whole wakes up to this fact and chooses to apply itself towards ridding itself of this animal character, we're screwed! Sadly, in 5,000 + years of teaching and warning regarding this issue, we've yet to catch on.