I'm not a fan of playing the defense, i.e. picking a "safe" candidate.

[Amimnoch]

7. Correlation does not equal causation.

Breaking each example down:

1. Al Gore: Basically lost as a result of a Supreme court decision. But more than that, we'd just had 8 years of Bill Clinton. Most forget that after 8 years of any party being in control, it's EXTREMELY rare that the same party wins a 3rd term for change of guard. Not since FDR has the Democratic Party held the office for more than 2 consecutive terms, and on the Republican side, only George HW Bush (a "safe" candidate I'll note) has broken that same trend.

2. John Kerry - Just as it's very rare for a party to retain the presidency after 2 consecutive terms, it's almost as rare for an opposing party to oust an incumbent candidate running for their second term. George HW Bush was one example of a single term, losing to one of the "long-shot" candidates you'd mentioned. Considering other factors though, such as 12 years of Republicans holding the office, and the Ross Perot factor (he likely would have retained, and gone on to a second term without the 3rd party running of Ross Perot).

3. Hillary Clinton: Much like Gore above, 2016 wasn't really our year to win, as much as Republicans year to lose. We'd had 8 years of President Obama, and in #1 above I pointed out just how rare it is for either party to hold the Presidency more than 2 consecutive terms. I won't even begin to go into all the other factors such as her popular vote victory, or the contentious primary season in a year where, frankly, having a contentious primary was a stupid way to go.

4. Mitt Romney: Another candidate running against an incumbent. He was also running against a highly popular sitting president who had turned one of the worst recessions since the Great Depression around. See #2 above.

5. John McCain - Same as #1 and #3 above. 8 years of Republicans holding the office + downturned economy.

The Safe candidates mentioned also played into the longshots you mentioned, so I won't recap.