I've been wondering, is there any explanation for why Cyrodiil isn't a jungle in ESO? Did they just twist the lore a bit for the convenience of having forests and plains as a battlefield, or is there an actual reason?

If I recall correctly it was a Transcript Error, it was never a jungle. Some scholars just wrote the wrong thing.

The same reason why it isn't a jungle in Oblivion: http://www.imperial-library.info/content/many-headed-talos, past, present and future.

"'You have suffered for me to win this throne, and I see how you hate jungle. Let me show you the power of Talos Stormcrown, born of the North, where my breath is long winter. I breathe now, in royalty, and reshape this land which is mine. I do this for you, Red Legions, for I love you.'"

But maybe you prefer a more mundane explanation. http://www.imperial-library.info/content/cyrodiil: "My studies indicate that the use of the phrase "endless jungle" to describe Cyrodiil appears to be an error in transcription. Close study to the original, badly faded manuscript reveals that the phrase was miscopied, and should be more accurately rendered as "extensive uplands." The adjectives "an equatorial rain" as applied to the Nibenese forest to not appear in the original manuscript at all, and I would posit were added by the scribe in support of his previous erroneous use of "jungle." Lady Cinnabar of Taneth of course takes issue with this exegesis, but the flaws in her methods of scholarship have been well-documented elsewhere."

So the 2 explanations are: - The power of CHIM changed Cyrodiil in the past as well, causing there to be reports and memories of a jungle-Cyrodiil even though in that branch of time it was never there. I guess we best don't start talking about how it's not possible for there to be memories and reports of something that never happened, time as we perceive it is just a b****.

- The report of Cyrodiil ever being a jungle is a transcript error and it always has been like it is now. This seems odd to me though, as the story of Talos using CHIM to change Cyrodiil's climate just came out of nowhere if this is the case.

There's one more possible explanation:

ESO takes place during a dragonbreak and Time is all broken, causing Cyrodiil to take on its future appearance and climate.

Keep in mind that http://uesp.net/wiki/Online:Ruminations_on_the_Elder_Scrolls has also found its way to Planemeld Tamriel, but the dating of the book is brushed off as a "transcription error."

Weird time shenanigans, yo.