Because the signature of "John Hancock" was/is the most prominent on the Delaration of Independence and Articles of Confederation. Being that its the most prominent it is the one that stood out and everyone remembers, so when someone refers to John Hancock his signature is usually the first thing that comes to mind and therefore one is being asked to sign their name.
John Hancock's signature is the largest.
£18 English pounds
"John Hancock" is presumably the problematic term in that phrase, the rest of it is pretty straightforward. "John Hancock" is slang for "signature", coming from the very prominent signature of John Hancock on the US Declaration of Independence. The John Hancock Life Insurance company had a commercial jingle featuring the line "put your John Hancock on the John Hancock".
The typical meaning for John Hancock is your signature. So when someone asks you for your "John Hancock" they are asking for your signature.
John Hancock, his signature is also the largest on the document
Signature - 1981 John Ehrlichman was released on: USA: 1982
John Lennon Signature Box was created on 2010-10-05.
The length of John Haines signature 1750el boat is 2.4 metres.
John Adams has the smallest signature on the Declaration of Independence.
John Hancock's signature is the largest on the Declaration of Independence.
In the related links box below, I posted the wikipedia article on John Hancock where you could see his signature.