Why More Debates in the Primaries than in the General Elections

A random thought popped into my head… why are there so many debates for the party nominations, but so few among the actual nominees from the two parties?

Off the top of my head…

1. A lot of people running for the party nominations means all of the candidates actually have to show up… if one candidate doesn’t show up, there’s still a debate.
2. The grass roots wants an up close and personal look before they coalesce around one of the candidates in the primaries. By the general election, they have coalesced.
3. There isn’t much to gain and a lot to lose by going to debates in the general election. By then, everyone knows the positions of both candidates… so there isn’t much of a message to get across, but the opportunity to commit some sort of gaffe never goes away.

Any other reasons?