It's not a huge deal anyways since there's loads of forest. If Cyrodiil was changed to a desert or something, there'd probably be more interest on the subject. Jungles and Forests are almost the same thing, right?
It's different jungle is untamed and is hard to navigate, forest on the other hand is not.
The better question would be what happened to the Imperial City? why was Sutch whiped off the map? and what happened to the culture and politics?
However, things would have been alot different had it stayed the old way, so I'd say its a pretty huge difference between jungle and forest.
Brief History predates Beth's retcon, which doesn't come around until Oblivion. Up until then, as far as we knew, Cyrodiil was still a jungle.
What's sutch? but by the time Brief history is written, Tiber has already changed it right? It was written after Tiber became Emperor after all and when did exactly Tiber changed the landscape?