Why garden and not whole world?

[Belacqua]

It's the only rationalization that makes them feel good.

Assuming that this is the cause would be the easiest answer, and would allow you to maintain a negative view of Christians without doing any further research. In that, it resembles the way bigots think.

The felix culpa also ties the Old Testament story into equally ancient traditions of katabasis literature. This is an important trope in myths in the Mesopotamian, Greek, and Roman worlds. In many cases, it symbolizes the need to leave one's place of origin and undergo hardship in order to mature fully. 

I don't know when it becomes common to see the Christian story in this light. Dante doesn't see things that way, though others may have before him. Milton certainly believes in it, and that made it an essential trope for poets after him, especially the Romantics.