Why do a lot of black people have flat noses?

Breathing in hot air was the main reason they have flat noses. You want to take air into a flatter area in order to cool it down before it enters your lungs. A flat area spreads out the hot air, thereby cooling it down faster. You want a larger narrower nose to take in cold air to heat it up slowly before it gets into your lungs. A narrow nose does that. Humidity and dryness plays less of a role, because a cold climate can have dry air. A hot climate can have either humid or dry air, which changes frequently due to cyclic changes in any one area. A prominent nose bridge however would burn too rapidly in the sun. So that is another good reason. People around the world all have different kinds of noses and there isn't anything offensive about all the different noses in the world. It's a nose. We all adapted to our climates.

The most common explanation for nose shape/size has to do with environmental factors (temperature/humidity/etc.) related to the function of the nose, which is to warm, clean, and humidify the air you breath, as well as helping you to smell and taste.

What people typically refer to as a 'flat nose' is actually a wide circular nose.

You are describing an equatorial nose. Continuous warm air to breathe, 24/7, no need to warm the air with long nose/nostrils, as found in northern climates.

Their distant ancestors lived in very hot and humid climes, so a wide nasal aperture can effectively humidify the incoming air. It is mostly an adaptation to climatic conditions.

I think it is because. Black people originate from africa where there is alot of sunshine so to stop there noses getting burnt they evolved to lie as flat against the face as possible to try and avoid the sun shining down on it as much.

Because of their regional conditions

Good question... I can't think of a hypothetical answer.