Partially in response to your other post as well, I've also seen it hypothesized that the collision that produced Earth's moon stripped away most of Earth's atmosphere, the idea being that Venus' atmosphere may actually be the "normal" atmosphere for an Earth-sized planet.

As usual, the core problem is just that our data set is too darned small. Is our Solar System mostly normal? Is our Solar System a bizarre, billions-to-one odds freak case? We don't really know.