We have a unique driving culture here. We love to drive, and we like doing it in our own cars. There's also the issue of geography. In Western Europe, traveling by car between cities, or even countries for that matter, takes a matter of hours -- which isn't always the case in this big ol' nation of ours. BlaBlaCar's success here in the US of A hinges on it finding a niche of users on the East and West coasts, where drives between popular destinations don't take a full day and a half.
As stubborn Americans, sometimes it takes us a while to recognize when we're sleeping on something really great. Look at Airbnb and Uber -- before they became the behemoths they are today, they had to clear some pretty significant hurdles: convincing Americans to feel comfortable handing the keys of their homes over to complete strangers and hopping in the backseats of random people's cars. But now that we're neck deep in the sharing economy, a service like BlaBlaCar may be uniquely suited to succeed here. Road tripping with complete strangers? Why not, dude.