I don't think that's true in and of itself. The election cycles have been 4 years since the founding of the country, but this level of short-term thinking has, I think, gotten worse over time.

Personally, I think it has to do with how quickly information can be transmitted nowadays, coupled with less trust in government than there was a generation or two ago. But maybe I'm wrong and someone with a firmer grasp of history can correct me by showing that this has always been a problem.