Because the entire biotech market was down and stocks typically trade with their sector. So the question is why on this day did it go up when nearly everything went down? REGN trades at 17x's sales. VRTX trades at 54 times sales. So the entire stock market agrees with you that the company is valuable. But why, in the absence of news, on a day of below average volumn, why did it go up when everything else went down?