I am not from the US, so I have to draw my "Sorry, I don't know" card here, but I really cannot believe that there were only bad candidates - It isn't impossible, but without proof it seems highly unlikely, i.e. an edge case.

Extreme variations are more likely the smaller your sample size is, considering America has a two party system they have a very small sample size causing it to be prone to extreme variation. Examples: No women candidate, no candidate that is not a politician, no candidate that favors alternate economic models, etc etc. This makes no "good" candidates according to a particular individual's view highly likely.

Nor am I. My opinion is that most of these guys are interchangeable, and (of the elite subset they are part of) quite average.

"Underwhelming" is a good descriptor. I've yet to experience someone describable as "good".