Because the companies behind SOPA are using international trade agreements as a backdoor to pass SOPA-style laws.
SOPA's supporters are pushing two agreements: ACTA and TPP1. ACTA would criminalize users, encourage internet providers to spy on you, and make it easier for media companies to sue sites out of existence and jail their founders. Sound familiar? That's right, ACTA is from the same playbook as SOPA, but global. Plus it didn't even have to pass through Congress.
Or rally and protest if you live in Europe, just get the word out.