Why did some delegates refuse to sign the constitution?

Elbridge Gerry, George Mason, Edmund Randolph
Three delegates to the Constitutional Convention in Philadelphia (1787) refused to sign:

Elbridge Gerry (MA)

George Mason (VA)

Edmund Randolph (VA)

Other delegates who were only there for portions of the summer were also not "Signers". People like Robert Yates and John Lansing from NY, for example.