Not to nitpick, but wifi is radio. The propagation of radio waves depends on their frequency. The frequency wifi operates on is in the GHz, which mostly is line-of-sight only. This is not always practical. See [1] for an overview of frequency and propagation.

[1]: http://en.wikipedia.org/wiki/Radio_propagation

Ever tried to communicate with wifi at a distance longer than 50 meters? Yeah. You won't get good results.