The reason given is esophageal cancer, but doubtless the challenge of Jackie Speier in the primary, where she was leading by 30% in the polls, had a lot to do with that too.

It comes down to the fact that he was a very strong hawk on Iraq, and any Iraq Hawk deserves to lose their seat.

FWIW, part of his statement is very disingenuous.

It is only in the United States that a penniless survivor of the Holocaust and a fighter in the anti-Nazi underground could have received an education, raised a family, and had the privilege of serving the last three decades of his life as a Member of Congress. I will never be able to express fully my profoundly felt gratitude to this great country.

Of course, Leon Blum (France), Bruno Kreisky (Austria), both of whom were holocaust survivors and not in America, and, of course, Menachem Begin in Israel were all PMs and Holocaust survivors. (H/t The Reality Based Community)

This kind of ahistorical American exceptionalism is de regeur in US politics, and dangerous, because it leads to things like invading Iraq.