Why weren't past primaries more crowded?

[Karpatsky]

People talk about how unprecedented the 2016 Republican and 2020 Democratic primaries were/will be in terms of number of candidates running - but why didn't more candidates run in the past, given the number of theoretically national figures is the same? Are parties just losing their ability to 'clear the field'?

[darklordoftech]

Keep in mind that past primaries were often more crowded than people remember. For example, John Connally ran in the 1980 GOP primaries.

[MarkD]

The 1976 Democratic primary was quite crowded, with about fifteen declared candidates. Besides Jimmy Carter, there was also Jerry Brown, Morris Udall, George Wallace, Frank Church, Henry Jackson, Robert Byrd, Fred Harris, Birch Bayh, Lloyd Bentsen, Terry Sanford, Walter Fauntroy, Ellen McCormick, Sargent Shriver, and Milton Shapp.

[Del Tachi]

Because we only tend to remember the winners/big names.  As has been mentioned, the 1976 Democratic primary field was very crowded.

Here's a summary for both parties # of "major candidates" in open races since 1960:

Democrats:
1960 - 7
1968 - 8
1972 - 15
1976 - 17
1984 - 8
1988 - 13
1992 - 7
2000 - 3
2004 - 10
2008 - 8
2016 - 5

Republicans
1960 - 4
1964 - 9
1968 - 18
1980 - 10
1988 - 8
1996 - 11
2000 - 12
2008 - 12
2012 - 8
2016 - 17

Prior to the 1970s, it's hard to exactly know the number of candidates running because 1) lot's of "favorite son" candidates and 2) delegate allocation rules were quite different, and candidates who never officially announced a campaign often won a significant number of delegates (i.e., Rockefeller in 1960 and 1964).  I included all favorite son candidates who held federal office as well as any candidate who won any significant number of delegates.  By this metric, the 1968 GOP field is actually the largest primary field in American history.

I would additionally posit that the increase in the size of primary fields we've seen over the past half-century (GOP has a positive trend; Dems have a near-zero trend, but that's probably due to uncompetitive open primaries in 2000 and 2016) has a lot to do with the rise of the 24-hour news media.  With some much airtime, candidates are almost guaranteed to get a significant amount of coverage.  This can be useful for candidates hoping to use a presidential run as a way to make money/gain recognition.