One theory is that in temperate environments, there are alternating harsh and mild seasons, so in the mild seasons people can PLAN (that's the important word) for the harsh ones. This need to plan develops technology/culture/brain cells.
In tropical climes, where the weather is pleasant year 'round, there's less of a need to plan because abundance is everywhere.
In environments of temperature extremes (very cold or very hot/dry) the emphasis is more on day-to-day survival, thus there's less slack time to plan for manufactured abundance/technology/culture etc.
Thank you, thecodont. I was having trouble expressing my reasoning. It’s merely a theory but it seems to me to be a good one.