SOPA hasn't spread around as much as people might think it should because the groups who are the type of people to worry about things such as the state of the Internet spread their information and worries in very insular ways. In other words, we geeks are very inwardly-focused when it comes to who we talk to. And it's hard to get our problems to translate to other people, and mean the same to them as it does to us. Not everybody spends their days on message boards, and if they are they have very different worries from those who don't. Even though SOPA can and will affect them if passed, it's just hard to bring them to terms with that. They don't see the Internet the same as we do. Plus we're just usually not on good enough terms with those people to have casual conversations like that anyway.
Plus, so far the Internet has proven itself to be rather indomitable. Nothing has legitimately threatened it or succeeded, at least in our corner of the globe. Until something happens that proves it's vulnerable, it's really hard to convince people that something isn't indestructible. So even those who might have heard about it may not have shared because it's so hard for them to accept that there's something out there that might actually change this status quot we've gotten so used to these last two decades.