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

Why? Try these on for size:

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.”

—Alexis de Tocqueville

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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:

—Thomas Jefferson

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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.

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

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

Joe Biden is not the best president we had. That would be John F. Kennedy. How is voting for Donald Trump any worse than voting for Joe Biden?

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.

—Benjamin Franklin

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.

What made you recently say to yourself, “Wait. Really?”

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

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

—John Adams

Why is it so hard to date nowadays?

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.