Read this: In 1970, Kerry had considered running for Congress in the Democratic primary against hawkish Democrat Philip J. Philbin of Massachusetts's 3rd congressional district, but deferred in favour of Robert Drinan, a Jesuit priest and anti-war activist, who went on to defeat Philbin. In February 1972, Kerry's wife bought a house in Worcester, with Kerry intending to run against the 4th district's ageing thirteen-term incumbent Democrat, Harold Donohue. The couple never moved in. After Republican Congressman F. Bradford Morse of the neighbouring 5th district announced his retirement and then resignation to become Under-Secretary-General for Political and General Assembly Affairs at the United Nations. The couple instead rented an apartment in Lowell, so that Kerry could run to succeed him.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): Why did Morse retire?
to become Under-Secretary-General for Political and General Assembly Affairs at the United Nations