[publius911]

Few people will ever see this expensive commemorative coin.
Why do they use the year 1792?

That's the year the original complete Constitution with Bill of Rights was finalized, but not yet completely ratified.

[Repeal The 17th]

In 1792 George Washington was being elected to his second term.

According to the US Mint’s website...
1792 was used because that is the year the US Mint (in Philadelphia) was established.

“The U.S. Mint is celebrating its 225th anniversary
by minting a commemorative coin which depicts
Lady Liberty as an African American.”