Because they, the publishers, make more per unit on hardcover sales than they do with other sales per unit. (madgeniusclub.com)
I * know * we're not the only ones, so even if hardcover sales have (or do) increase, they would have have increased far more without e-books. (kriswrites.com)
This has not directly influenced hardcover sales, but trade paperback books are on the decline. (goodereader.com)