Kerry chose to hide this fact so that: 1) He would not upset his power base, the virulently anti-Israeli Left in America, and 2) He could continue to pretend he was Irish to get re-elected by the simple minded Massachussetts lemming voters.
2) is quite true. Kerry wouldn't have gotten elected if not for the Irish connection. His early opponents portrayed him (quite accurately) as a distant and aloof WASP and only Kerry's Irish name saved him.
1) is much less likely though. How much support has Kerry gotten from the "virulently anti-Israeli Left in America?" Really, how much of a force are they in the Democratic Party? What real evidence do you have? When Wesley Clark and Hillary Clinton "discover" Jewish connections to advance their careers and Democratic candidates routinely pledge themselves to support Israel, how seriously can anyone take that contention?
Some Likud loyalists would claim that anyone who isn't 100% behind Sharon is "anti-Israeli," but given that many Israelis and American Zionists fall into that category themselves, we needn't make that any sort of orthodoxy or paint ourselves into a corner.