Sometimes, we do use the Doppler shift caused by a planet pulling the star towards or away from us. That works as long as the orbital plane is not perfectly perpendicular to us. However, it only works for large planets (and reasonably sized stars) that have sufficient gravity to make the star move fast enough to see the light shift red or blue. An earth-mass planet would be undetectable compared to a Jovian planet.

The transit method requires the orbital plane to be perfectly on axis so we can observe an eclipse as the planet passes between the star and our telescope. This is rare, but works for even small planets, as long as the angle is right. And if one angle is right, it's likely that others will be close enough as well; a large planet can also be observed. In systems that are at the right angle, Kepler often found several planets:

https://en.wikipedia.org/wiki/List_of_exoplanets_discovered_...