Probably for the same reason you see the names Kennedy, Clinton, Bush etc. repeat itself. It's one thing to say it's the voters that decide in the end, but to win a presidential nomination you need a vast number of campaign staff all over the country to work for you personally. Maybe you have some fame in your city or in your state, but what does the average voter in Nevada know about politicians from Kentucky? Very little I would think. So unless you're very rich, very famous, have family backing or have been working your way up the political system for decades you don't got the support network you need.
In many other countries it's more of a political process, you need to convince other politicians in the same party that you're the best candidate to put forth and then you have the party's resources at your back in the public campaign. What you need is mainly to be an attractive candidate and they bring the marketing budget. In the US it's a massive personal effort to make a bid for the presidency and for most it's a once-in-a-lifetime shot. So you keep putting it off until the end of your career and it becomes a "now or never" shot but a pretty strong one at that. I wonder what the statistics are for first vs second+ run for nomination/presidency.
Of course there are exceptions for the rule, like Obama "should not" be able to pull that off after three years as a senator but that guy could probably sell sand in Sahara. Also even though it's an election every four years you'd be nuts to challenge the nomination of a sitting president, plus you have to consider the possibility the voters are fed up and think the grass looks greener on the other side so realistically good chances might only come every 8-12 years. It's better to go all in once and succeed than try and try again. It hasn't worked for Clinton, it hasn't worked for Bernie, maybe it's a human flaw but I think people are likely to go with this election's fresh breeze over the candidate they rejected years ago. Even if they're a better candidate now.