User blog comment:Lokiwg/Summoners Rift/@comment-5331854-20140414154119

Reason Lore-wise:

The place where the Rift is has huge amounts of magical energy in great concentration and it's also a place where many battles have been fought in the Rune Wars, making it an obvious choice because of it's historical, military and magical value. The wild monsters of the place also makes a great addition for those fights, with Baron Nashor being someone who naturally dwells in the river.

Reason Design-wise:

It's based on the original DotA-map from Warcraft 3, which had a similar design, mainly because you can interact with trees in Warcraft 3 (and therefore in DotA and DotA 2). It would also be hard to replace the brushes with something that would fit in an arena that is Roman Coliseum-styled.

Plus, sand and stone walls are boring to look at.