They originally put it there because it's easier to locate apparently:

> The Fn key was originally placed by the ThinkPad designers in the lower left hand corner to make the key easier to locate when using the keystroke combinations. There was a rationale. This is especially handy for turning on the ThinkLight in the dark. Aim for the two extreme corners.

https://web.archive.org/web/20111115202457/http://www.lenovo...