Why would a smaller orbital radius be easier to detect?
For a Doppler shift measurement, you get a bigger amplitude of observed wobble (larger velocities) for bigger gravitational force, and this force is larger when the planet is closer to the star. (Note that for astrometric measurements, for which you are actually looking for stellar displacement directly, a larger orbital radius is in some ways easier to see, because it means that the center of mass of the star-planet system is more displaced from the center of the star.
