According to one classic theory, the reason is simply that there is more habitable space around the equator than at the poles. On the face of it, this seems to make sense.

据常规判断，赤道地带比两极有更多的栖息地，表面上看这个原因很是合理。

youdao

One of the two dates each year when at noon the Sun is directly overhead at the equator and day and night are of equal length.

一年中出现两次的日子，这一天中午太阳直射赤道，并且白天和晚上有相同时间长度。

youdao

At the equator, where the Earth is widest, fifteen degrees stretches about one thousand six hundred kilometers.

VOA: special.2010.07.14

But in measuring distance, one degree shrinks from about one hundred nine kilometers at the equator to nothing at the north and south poles.

VOA: special.2010.07.14

"It means if you take an area one degree by one degree, latitude by longitude, which at the equator is roughly 100km by 100km, you can take any two points in the DEM and the error between the two points is less than two metres, " he told me.

BBC: Mapping Earth's surface in 3D