First of all, if we define being “in power” as having the president be from your party, there has been one period in which a party was in power for 12 years. This was the Reagan-Bush era when Reagan had two terms and George Bush, Sr. had one. In addition, we have to realize that the Republicans have held the White House for 7 of the 11 presidential terms since 1968 (the Democrats only got one term from Carter, two from Clinton, and one from Obama pending the upcoming election).
That said, it is true that control of the presidency has changed hands a lot. This is because the population of the US is split in ideological terms and no major crisis has happened to get more people on one side or the other. The only time of sustained dominance by one party since 1900 was the Democratic dominance from 1933 to 1953. This was driven largely by the Great Depression and was sustained only by very close victory by President Truman in 1948.
Since 1952, there has been no big event that would have turned people against one party or the other. Meanwhile, the electorate is deeply split, making it very difficult for one party to keep winning for a long period of time.