Because outside of the in-game books Oblivion makes no contribution to elder scrolls lore.
That's completely not true, as long as the game tells no stories in the world, it contributes to the lore, some fans just prefer to think otherwise, because they are arrogant enough to think that what they want the lore to be is substitute for Bethesda's canon.
Anyway, the real explanation for what happened to the jungle is the whim of Todd Howard, who thinks that players want familiarity, not originality, he felt that Cyrodiil as a jungle wouldn't be familiar to players, so he decided it should look like a more boring version of Middle Earth instead. We just got "Talos did it" as a half-hearted handwave to explain such a design choice. I don't like it, but to just boycott new lore because you don't like it is foolish and doesn't change anything.
And such an amazing gift! A single line: "...through the familiar Cyrodilic countryside. Past fields of wildflowers, gently rolling woodlands, friendly hamlets". Clearly, fields of wildflowers, gently rolling woodland and friendly hamlets only exist in Pastoral England Cyrodiil.
To be fair, when I read a fantasy book and read those words, something along the lines of Cyrodiil is usually the image my mind forms, but that is in part because I already know that it's set in your usual Pseudo-Medieval European Fantasy Land, in reality, it's far from indication that Cyrodiil looks like that.