The two political parties hold conventions to nominate their Presidential candidate. These are huge affairs requiring large blocks of hotel space and a venue, usually something the size of a Basketball stadium. That all requires a lot of advance planning and therefore a commitment to a date.
Presumably there is an advantage to either going first or second. It may be that first impressions matter the most and so going first is desirable. Or it may be that people remember the most recent convention more vividly so that going second is better. Whichever it is, the incumbent party has an advantage in the convention timing game.
The incumbent party already has a nominee. The convention doesn’t accomplish anything formal and is really just an opportunity to advertise its candidate and platform. The challenger, by contrast, has to hold the convention in order to formally nominate its candidate. This is not just to make formal what has usually been decided much earlier in the primaries. Federal law releases the candidate from using some campaign money only after he is formally nominated.
So the incumbent has the freedom to wait as long as necessary for the challenger to commit to a date and then immediately respond by scheduling its own convention either directly before or directly after the challenger’s, depending on which is more desirable. The fact that this year the Democractic National Convention followed immediately on the heel’s of the Republican’s suggests that going last is better.
I haven’t seen the data but this theory makes the following prediction. In every election with an incumbent candidate the incumbent party’s convention is either always before or always after the challenger’s. And in elections with no incumbent, the sequencing is unpredictable just on the basis of party.