it's a possibility that these senators all ran against incumbents. Goldwater would have made a great president, but Johnson was riding on Kennedy's coattails. Goldwater also challenged Johnson to a debate, and Johnson declined, which if it happened, Goldwater couldv'e done better. Bill Clinton was a controversial character, but he beat Bob Dole, in part because of the personalities of both candidates, partly Bob dole was 72 years old. Kerry didn't appeal at all to any of the 2000 swing states that Bush hardly won. He would've done better if he had let Edwards do more campaigning. Also, like Bob Dole, he's boring as dirt to listen to. I think the main thing is in all three cases, they just didn't have the appeal their opponent did.
Look how many sitting Senators
lost in the primaries. Leiberman and Edwards in this cycle; McCain in the last one. Specter and Gram in 1996; Harkin and Tsongas in 1992. None of these were running against incumbents.
The Senate is not a good place to launch a presidential campaign.