It's because BSG is pushing this one to eleven.
If a planet's rotation on it's axis is too long it get's too much radiation during the day, most likely not enough during the night (even if the moon reflects a fraction of the radiation).
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If a planet's rotation on it's axis is to fast than the surface is getting too little radiation all around.
By definition the planet is getting the same dose of radiation in both cases, just at a different day/night frequency. I think you are postulating that the global climate will change linearly from what we have here on Earth to...
On a planet (say) with a tidally locked rotation (meaning one side is always facing the sun, much like how one side of the moon is always facing Earth) Than the only place life could theoretically survive is where day and night meets. Though even then it would be a challenge.
... as the rotation of the planet slows to zero. In reality, even a tiny rotation may be enough to sustain a thriving biosphere, especially if there were large oceans supporting a well-developed and turbulent atmosphere. Look at Venus, it rotates less than twice per orbit, but you don't see temperatures on the dark side falling to frigid lows, nor temperatures on the day side getting wildly hotter than average (which is admittedly already pretty high!). The churning of the atmosphere around the planet distributes the heat from the light side to the night side and back, maintaining a near equilibrium. In fact, it's atmosphere actually rotates faster than the planet itself, almost once every 4 Earth days. Venus' only problem is that it has
too much atmosphere and of the wrong type.
It is only when a planet's rotation actually
stops that the wind patterns would change to cause the tidally locked situation you describe above. Really, meteorological phenomena and celestial mechanics is a lot more unpredictable than you make it out to be.