There is no one clear reason we know but there are some guesses people make.

• Locke wrote that “The necessity of pursuing happiness [is] the foundation of liberty” in his essay “Concerning Human Understanding" which could have been Jefferson's inspiration. 
• The south could've enshrined slavery if the central text of the goverment would claim to uphold property (though it does in the Constitution).
• Expressive and Easier to explain to the poor people of America