Because white people get preferential treatment every god damn day. Either you acknowledge race and gender or you go through great pains to take it out of the equation.
A good friend of mine is a female french horn player. Orchestras sometimes do auditions behind a screen with the applicants names anonymous to the jury. This is to counteract the sexism that exists in classical music, particularly in the brass sections.
My understanding is that YCombinator is very hands on and demo days are done in person? The person is judged, not just the product.
While I'm more in agreement with your line of thinking than lulzwat's, you're not really addressing the part he's disagreeing with. His proposition was that you counteract preferential treatment by trying to remove it, not by trying to balance it out with other preferential treatment. Even the example you gave supports that (_everyone_ is behind a screen).
Like I said, I do agree with your general sentiment because the situation is way more complicated and a given entity (YC in this case) doesn't have control of all upstream sources of preferential treatment to try and remove preferential treatment. It just seems like the way you phrased it didn't address the actual problem lulzwat perceived. As much as people like to pretend it isn't so they can sit on their high horse, this stuff can be complicated to get your head around (at first glance, it's 110% reasonable to say giving preferential treatment to black people seems just as racist as doing so for white people). It serves no-one to ignore someone's complaint and respond with genericism when they don't understand (or agree with) the nuances of this kinda stuff.
Where are you getting the idea that preferential treatment is being given? Y Combinator is opening it's umbrella to include more diverse walks of life, not giving any group preferential treatment.