It shouldn't be, but it is. This is a simple question of what our society is really like.
As cavemen and women, we were programmed by nature to find partners who appeared to be in good health. Good health meant surviving offspring meant preservation of our species.
That happens to be why we don't like the look of the obese or disabled. It's not because they are inherently ugly, it's because the layout of our brain chooses to evoke such a perception due to our natural history.