My understanding is that the Echo is very rare on the Source, and Krile was the only test subject with an Echo they were able to acquire. There are a grand total of... five known people in Aldenard who possessed the Echo during the events of the FFXIV story (WoL, Minfillia, Yslaye, Krile, and Mikoto).
Remember, there is a limited number of Amaurotine souls in circulation. Amaurot was just one city, and a large number of the population's soul are not in circulation in the Lifestream because they're tied up as Zodiark, and another portion of the souls are tied up as Hydaelyn. Furthermore, Aldenard is just one continent on the planet. The remaining Amaurotine souls could be reincarnating in other parts of the world, such as the New World and Merycadia. Thus, why the number of Echo users is ludicrously low. Aulus' research pretty much never progressed until Krile came along. The hundreds of bodies we see in Ala Mhigo are not Echo users, but normal people who were used as test subjects that Aulus was trying to clone Krile's echo on to.
Contrast that with the First. 90% of the world's landmass is uninhabitable. The entire world's population of souls is reincarnating in a very small area, but the number of Amaurotine souls has not changed. Thus, why there appears to be a far higher percentage of Echo users; more Amaurotine souls bunched up in a smaller area. That's why we see so many people awaken their Echo in 5.2 and 5.3.
The other thing is, I'm not too sure that the Resonant is wholly a clone of the Echo. Zenos manifested powers that no Echo user ever had, such as being able to use Ascian portals, though it's unclear whether or not that's something Emet-Selch did to Zenos, or a power of the Resonant.