Because the instructions say "Countries are as they appear in Countries of the World quiz". Have a look at the Official Country Names quiz for the actual longest, official, names, where your suggestion is top of the list for any letter, not just U.
Because the instructions say "Countries are as they appear in Countries of the World quiz". If we accepted official country names then, for the letter U, United States of America (21 letters) would be edged out by United Kingdom of Great Britain and Northern Ireland (45 letters).
Nice quiz. I've been curious to ask you something for a while: who's "we"? as in, "If we accepted..." You use this wording in many of your quizzes. Is there a team of jerry928s that submits your quizzes?
Could you accept answers with the same inputs as the countries of the world quiz? Examples: CAR and Democratic Republic of Congo would count for Central African Republic and Democratic Republic of the Congo.