For Manaus I think is because the city stands out. It's a 1,5 million ppl city just in the middle of Amazon Forest. They frequently use huge boats to travel and they are high industrialized (not as some foreigners could think lol).
Is it a hearty breakfast? Mmm, my favorite brand. Is it a four letter word that you can use in various ways that are mostly positive? Hell yeah.
I got stuck on the 5th most common guessed city in Brazil, and it took about 45 seconds!