Radon gas in earthquake prediction - Why Rn?
Uranium is found in low-levels in all rocks and soil.  Radon is a gaseous radioactive decay product of uranium.  As the uranium undergoes radioactive decay, radioactive radon is generated and trapped in the rocks that contain the uranium. 

The earthquake theory involving radon suggests that prior to the actual quake, there is some subterranean movement where rocks are crushed, soil is uncompacted and the trapped radon is released producing a pre-quake spike in radon concentration. 

So radon presents the following attributes:


it is present in all rocks and soil
if a rock is broken or if soil is disturbed, radon will be released
it is gaseous and air currents / thermal gradients will carry it up to the earth's surface producing a detectable plume
it is radioactive, this makes detecting small amounts or small changes in radon concentration relatively routine due to well-developed and very sensitive methods for detecting and accurately measuring radioactivity.
radon has a very short radioactive half-life, a bit under 4 days.  Being a gas and having a short half-life is very useful in terms of measuring radon emissions.  If a radon emission spike occurs, the gas will dissipate quickly and after about 10 half-lives (40 days) normal background levels of radioactivity will return.