The Democrats actually managed it back after the Great Depression. Three terms of Franklin Delano Roosevelt followed by one of Harry Truman.
So really, it's just uncommon in the last sixty-odd years.
The problem with that analysis in this particular case is that it is an attempt to normalise what is a fundamentally abnormal election (which is what a lot of people unfamiliar with the historical and current European and international politics generally are guilty of doing through ignorance). This wasn't a normal change of the guards, and is part of a trend that we are seeing across the world. A charisma candidate that is frankly unsuited for office, with almost no real institutional support from any recognisable political peers, and a disdain for the norms of liberal democracy just beat in a head to head contest a candidate that is fundamentally a normal, if flawed, candidate.