In addition to the unseating of an incumbent President being very uncommon, there is a related fact. In the post-war period, the pattern in Presidential politics has been for one party to hold the White House for two terms or eight years. That pattern has been broken twice before now, both times favoring Republicans.
Think about Truman leaving office in 1953. Then:
R—Eisenhower 8 years
D—Kennedy/Johnson 8 years
R—Nixon/Ford 8 years
D—Carter 4 years, the first break in the pattern
R—Reagan/Bush1 12 years, the second break
D—Clinton 8 years
R—Bush2 8 years
D—Obama 8 years
R—Trump 4 years. A break in the pattern and the first favoring Democrats. Another first for Trump!!!