I think I can guess why the first quiz was chosen as the world leaders quiz is overly dense in answers and open to nitpicking. Less clear as to why the Stewart quiz wasn't chosen but perhaps it has something to do with his affiliation with the site? QM, happy to get any clarification on this!
1. Too complex
2. Low quality quizzes (there is a quiz with 86 noms where all the answers are Zlatan)
"The Quizmaster exercises the right to choose a quiz other than the winner."
And then maybe since QM didn't want to feature the random merged asian countries quiz, he decided to feature a different of Stewart's quiz (the A-Z Capitals one).
It would take a lot time to fix, which I don't have.
It also took me about 45 minutes to fix the quiz which was featured.
Like Lucky said, Quizmaster has the right to choose.