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

Why? Try these on for size:

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

—John Adams

Why do creationists ask for proof of evolution and then ignore the answers?

—Thomas Jefferson

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

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.

Asian Shares Have Tepid Start Before US Jobs Data: Markets Wrap - Bloomberg

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.

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

—Alexis de Tocqueville

The Smallest Dinosaur Ever Discovered Was Just… - The Daily Galaxy

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

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.”

Are Latter-day Saints watch the Secret Lives of Mormon Wives? And if so, what do you think?

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.

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

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

Is there any new American thing?

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.