Because they don't dare make an honest argument. If they were saying what they believe, it would go roughly this way:
"The internet threatens our longstanding control of information and communications, and that is simply unacceptable. Therefore, it is essential to curb the utility of the internet for everyone else."
Some Sopa opponents have their own semi-blind spot. We are sometimes reluctant to acknowledge that, despite the copyright industry's blatantly bogus claims about losses from infringement, there are at least some losses. But copyright holders have always suffered some losses to infringement. They, and society, lived with the trade-off. Information could spread more widely, with great benefits to all, despite some losses.