Why is the word "democracy" not in the preamble of the US Constitution?

Why? Try these on for size:

To slaves: “You can forget your notions of freedom. We voted fair and square and your side lost.”

Majority-rules democracy is the way that majorities run roughshod over minorities, destroying rights and all ideas of equality under the law. No fair-minded person wants anything to do with democracy.

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—Thomas Jefferson

Another tendency, which is extremely natural to democratic nations and extremely dangerous, is that which leads them to despise and undervalue the rights of private persons.

—Benjamin Franklin

How can the democrats say Mr. Trump is bad when he is already fixing this country again and he's not even president yet?

The republican is the only form of government which is not eternally at open or secret war with the rights of mankind.

When the people find they can vote themselves money, that will herald the end of the republic.

To Indians: “Pack up what belongings you can carry and start walking to Oklahoma. Your fertile lands are ours now. We voted fair and square and your side lost.”

Can we trust the Bible when Constantine and the First Council of Nicaea took out many books of the Bible and altered existing translation by removing things?

—John Adams

I do not say that democracy has been more pernicious on the whole, and in the long run, than monarchy or aristocracy. Democracy has never been and never can be so durable as aristocracy or monarchy; but while it lasts, it is more bloody than either.

Meanwhile, when the Democratic Party formed in 1828, it was with a Tory outlook keen on robbing others of their rights:

What is the most interesting question you can ask to get to know someone?

—Alexis de Tocqueville

To blacks: “You can’t drink from that water fountain… eat in this cafeteria… ride this bus… go to this school. We voted fair and square and your side lost.

[with republicanism being the rights-protecting form of governance afforded us by our Constitution]

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In other words, our Founding Fathers and other Whig intellects of that time well understood that democratic methods are the surefire means to do serious damage to the rights guaranteed under republican self-government.