Why?

Can't be because of orbits, since those don't work like in our world. Also, since sunlight is basically pure magicka, and as we well know, magicka can be cold (well, not magicka itself, but you get it) as well as warm.

Because that's how the gods designed it(It's a relavent excuse here) and Magnus did design the world.

I'm aware. The question is why.

Because Lorkhan/The original 8 Divines/Devs said so.

Hell, until Morrowind (at least, if not Redguard) the sun rose in the west and set in the east.

This is one of the few cases where the ultimate answer is basically that http://tvtropes.org/pmwiki/pmwiki.php/Main/AWizardDidIt There's no explanation as to why it's like that. It just is.

Maybe Nirn's equator is located under Tamriel and right through Summerset?

Because Skyrim is close to the cold Atmora, where as Alinor is close to the warm Pyandonea.

Yeah, but my point was why is Atmora cold and Pyandonea warm.

I know they just are. I was kinda hoping for a lore-based answer, since everything else seems to have one.

The sun does seem to work in the same way as our own,Magicka when not bent by Mortals is pure energy.
Maybe the Et'Eda who make up nirn of certain areas affect who the magicka affects one region to the next,The ones near Skyrim,High Rock and Atmora are more inclined towards frost while the ones in Cyrodill,Black Marsh and Alinor are more inclined towards fire.
Also this
The Gods in the TES universe are bat [censored] crazy..Thats why.