The only correct answer is if you have a vested financial in the election?
Never were my home islands as featured in the news as they were when the Iraqi invasion was decided there. United States role in the World Wars changed the shape of Europe and the World. How isolationist does one have to be, being an American nonetheless, to not realize the impact of their own country in everyone else's lives.
Also, contrary to most Americans, many Europeans of younger generations see themselves as citizens of the world, and die-hard patriotism is an outdated idea to many. I not only care about what goes in America for the practical reasons cited above, but also get pissed off when some xenophobic leader is elected in Philippines or Hungary. Obviously I don't follow it as closely as in America - the impact is proportionally smaller - but the world is growing ever closer (much due to technology), and general cultural fads and ideals spread so massively that one can only view the world in a global context. That "PR gone mad" thread by Pogue, which sounded particularly American at the beginning, well, I see that in our colleges already, and we don't have the cultural similarities England has to justify that. Point is, the whole world is culturally closer.
Even excluding all international policy or economic questions, there are loads of reasons to feel invested in it. For a believer of Social State like me, for example, it would be a huge thing to see the country which most openly disdains it adhere to it, and it would, even if only by changing prevailing views, certainly protect our own social states from the threats they have. And that's not just because of my political leaning, obviously a libertarian would feel exactly the same.