Since the Democratic vs Republican elections started, a Democratic candidate won an open seat election only four times: 1856, 1884, 1960 and 2008. An average of one in every 40 years.
What is the importance of this information? I don't know. I am not saying that Republicans have structural advantage in open seat elections, that they have an advantage in 2016 because of the past.
Only a fun fact.
Basically, Democrats do better when they have a incumbent they can criticize. Makes sense and isn't a huge deal.
Why do the Democrats need to be incumbent or to run against an unpopular incumbente to win?
There were 40 Democratic vs. Republican presidential elections
17 Democratic wins and 23 Republican wins
The number of open seat elections was 16
4 Democratic wins and 12 Republican wins
The number of elections in which one incumbent was running for reelection was 24
13 Democratic wins and 11 Republican wins
There was a huge advantage for Republicans in open seat races