I believe that it is not simply the decision of leaving the EU that is the big news. It is the ideas and discussions that caused that decision to be made by the majority of the UK population (this article focusing in on the economic inequality portion of the argument) that are the most interesting and cause the largest reaction.

I should also mention that it is big news in the US because we are seeing similar sentiments from a portion of our populace and the foreshadowing of our own elections is, well, interesting.