
Because every single one of them is electable. Most citizens do not like Donald Trump, and do not like him hard. Even most of the ones that do like him don't care for him personally, because he's rude and childish, but they like how that gets liberals all worked up. Of the remainder who really, really like him, a significant percentage will fall down go boom or soil themselves on their way to the polling place.
So if you really like one of the leftmost candidates (formerly known as Eisenhower Republicans) but think you'd have a better shot with someone beiger who can't be pinned down to any position, think again. It doesn't matter who we pick, because the attack machine will be the same for any of them. If we nominated that flamer Jesus, he'd get the same treatment. The massively-endowed think tanks and focus groups that champion the Billionaire Protection Party never let the truth get in the way of a good tactic.
The eventual Democratic nominee will be called a socialist. Doesn't matter that there aren't any socialists running, or that people don't know what socialism is. Most of the ideas Democrats are espousing are very popular. You call the very same platform socialist, and the public turns on them. There will be very sophisticated memes out there proliferating, depicting the candidate as crazed or drenched in red or wearing horns, because that is how we choose a President these days. If you ask the average voter what a socialist is, you will be told that it is a person who wants to take all you have worked for and give it to someone who doesn't deserve it. That's actually what the billionaires are doing, but the Billionaire Protection Party has plenty of cash to devote to confusing you about these things.

So go ahead and nominate a real grownup. You might just as well. The public might even surprise you and elect them.